AIBU to feel uncomfortable calling my MIL's husband Grandpa?

He sounds like a nonce with the keeness to change nappies. No way would that be happening and tough shit if mil gets upset when she is told why - as if it's not obvious it's an off thing.

A man who's never had his own children wanting to change the nappy of his step grandchild 🛑⛔ a d that's before we even consider the other traits he has.

I had the same reaction to my MILs husband. Consequently we’ve never called him grand anything. He gets called by his first name and he gives us all the creeps.

In addition to what others have said, I would talk to dh and we would refer warmly to dhs dad and your dad every time they are around. ‘Dad would have loved to be here to see this, I remember how we used to…’ ‘have another mouthful sweetie, this is what grandpa Joe used to make your mummy!’
Creep.

It doesn’t really matter what your child calls him but I wouldn’t be allowing him to be involved in personal care. His behaviour would make me uneasy about him having unsupervised contact going forward.

It would be a cold day in hell before a non blood relative would be changing my child’s nappy! Wtf

You have been astonishingly passive in the face of this man who's a walking red flag OP. Please consider talking the NSPCC helpline for advice. I would personally cut all contact, but if you don't want to do that, then stop contact for a month then agree to meet only MIL in a controlled environment - a cafe or park. Start to impose boundaries because so far you have allowed MIL and her disgusting H to ride roughshod over you. If she refuses, then you don't see her. You should not be letting anyone else change your baby's nappy except his own parents and nursery workers. You need to wake up! If you do cut contact, please ensure that you talk to your HV or GP about the situation and why you have cut contact. They sound determined enough that they might seek a contact order to see your DC alone. You need to ensure that doesn't happen.

I won't comment on the nappy changing, others have done that perfectly well and hopefully the message is sinking in.

In terms of his name I'd be quite clear:

"Harold, I'm sorry you feel that way, but Dave and I have decided we're keeping grandpa for when we talk about our dad's who unfortunately aren't around to see little Timmy grow up. We expect our decision to be respected, even if it's not the result you wanted."

And if he kicks off..

"It's not our baby's job to paper over the cracks of the decisions you made in your life, you chose not to have kids. That's it."

I was all ready to be pro Grandpa based on my own family, but no.

What a gross person - having an affair with MIL and then living off dead FIL’s estate. He’s clearly going to wait it out for MIL to die and then take all her stuff as well. Foul. And he is wanting to just pretend he is actually FIL in being called grandad. I called my stepfather (who was actually a nice man) Grandad Chris.

He sounds very very weird wanting to change a baby’s nappy. I would absolutely not allow that.

You could cut them both off. They just sound awful.

Assert your boundaries. Trust your instincts.

What is your DHs take on this?
If he's siding with his mother and her husband, show him this thread.

Unless it’s Granny popping over to help so you can have a break/kip, anyone wanting to change a baby’s nappy (as in actually actively insisting they do this) is probably a massive weirdo! I have no idea why this red flag is being ignored, nor why you allowed it. Don’t leave him alone with your baby and he obviously isn’t Grandpa.

You and your husband both need to read up on grooming.

He sounds creepy and predatory and full of red flags. Your MIL will enable whatever he wants so you're going to have to watch her too. No all be time for either of them.

If he tries to block you from your baby, tell him to move, you are getting your baby. Don't let MIL or him use ownership language, their grandson, his grandson. He is MIL's husband, and he has no claim on your baby.

If he's become controlling with MIL, as relatives can you do a Clare's law request as well as Sarah's Law?

No decent man who hasn't even had his own kids to practice nappy changing on is going to go around insisting people let him change their baby's nappy. And rest of it doenst sound great either.

I agree with PPs about insisting on your boundaries. I have an alternative suggestion if you do not want to call him 'Grandpa'. In our family (more because my mother-in-law didn't want to be called 'Grandma' and step-father-in-law couldn't decide if he wanted to be Grandpa or not) we use nicknames for the Grandparents - i.e. instead of Jessica it's JessJess and instead of Robert it's RobRob. Like this it still feels like a 'special' name without using Grandma or Grandpa.

I’ve posted this before ; trust your judgement!
my daughter had a step grandad, very unpredictable; once my daughter hit 18 he sent her WhatsApp messages ; referencing that his willy is like a snake …. My MIL said it was banter . He was arrested as he failed to surrender any of his devices to police in a house search . Didn’t meet the threshold with CPS sadly and they were unable to say if they found anything sinister . When asked if the police officer would allow contact with her children if situation was reversed “ god no” was the reply.
my gut feeling for previous 16 years was right .
we are now no contact since , and if I could get my hands on either of them; I would do time for him !
to add; my daughter has autism, adhd, learning disabilities. The man’s a beast .
follow gut feeling every single time .

Nobody 'wants' to change a nappy. Hell, I'm a grandmother and I only change them to be helpful/if neither parent is around. When Gavin tries to (and apologies to everyone called Gavin) say that if he wants to help he can make you a cup of tea. If you refer to him by his first name that is what your DC will learn.

Do you know anything about his past, previous relationships, work history etc , and how he come to meet MIL ?

You haven't mentioned anything about your husband's feelings on this. Bedsides the obvious that this is your baby and you need to start setting some big boundaries, what does your husband think?

My mil remarried a coercive unpleasant man. We call him grandma’s husband and have photographs in the house of late Fil and refer to him as grandpa. The key is for both parents to be united and agree boundaries. The new husband made one inappropriate comment about my baby daughter, we put hard boundaries in place and funnily enough his interest in visiting completely evaporated. You don’t have to ignore your instincts to keep the peace.

I would avoid this creepy bloke as much as possible.

Keep that man away from your baby.

"Around the baby, step-FIL is very keen to play the physical role of grandparent. He wants to hold him, be the first to change him (hardly let's me change him when I'm around!), and play with him. He does not even let my MIL have much quality time with the baby. It's quite full on, to the point that it feels fake."

This is quite disturbing, particularly as he's already a man who's making you uncomfortable in your own home. You're going to have to be very strong here as no-one seems to have your back when it happens. You'll have to be very clear and exceptionally firm that this is your baby, he will not be changing nappies and that holding your baby is a privilege that you may or may not grant, according to him learning some humility, it is not a right.

Trust your instincts and let him learn his place in the scheme of things despite the opposition you're clearly going to face.

Strange, creepy, controlling and financially abusive - he sounds like a walking red flag. Call him by his name. Do not encourage your DC to call him anything else. Sounds like he's acting out some kind of fantasy, which is very worrying indeed.