FIL started being unkind to DGC

I’d never visit again, easy. But I would also have pulled him up on it.

Two separate issues here. I suspect your father is in cognitive decline. Second your son is being treated abysmally by someone in cognitive decline. Separate out the issues and deal with them separately. Don’t bring your son to his GF. That relationship is over now, it is too emotionally damaging for a child to deal with. Get your DH to bring his father to the doctor and raise some of these issues.

Please stand up for your son and don't let your FIL speak to him that way.
My Mil is a narcissist and I regret not standing up for my kids. I was always worried about confrontation. My MiL withdrew attention towards my dc when my sil (her daughter) had a child. I still remember when my ds who was 9 at the time, asked me what he had done to upset granny. I asked him why he said that and he replied that he didn't understand why she didn't want to see him anymore.
Thankfully we are NC (for a lot of reasons) and my ds is much better for being away from all that toxicity but i will never forgive myself for not standing up for them when I should have.

I wouldn't be exposing my child to that behaviour. It's incredibly damaging for children and I don't think your son is old enough to fully understand if there's something wrong with grandad to make him behave that way.

Dementia does sound like a possibility but this also sounds fairly selective so maybe he is just being mean and nasty.

Your husband has to speak to him and tell him to cut it out or you won't be visiting. Your husband can go alone.

Your poor son.
I hope you have done all you can to help him understand that his gf likely would not have meant what he said and is likely unwell (explain in as much detail as age appropriate).
You don't want him to internalise those insults any more than can be avoided. 😪

As soon as you said he was always a lovely , kind and gentle man then dementia popped into my had.

my gran was always the same and slowly she started saying horrid , mean and hurtful things which was the beginning of dementia. Eg you are too fat to wear that outfit, why are you still married, why does he put up with you. I was so so upset that my kind, caring granny could say such hurtful things!

at first we called my gran out on it , tried to make her explain herself but we soon realised when we did that she got upset because she said she would never say anything hurtful like that… which was previously true.

you need to protect and stand up for your child. He is 8.

Ah yeah it sounds like dementia for sure then.

He needs to see his doctor and it's definitely the right thing to stop visiting him as it doesnt sound your FIL is enjoying the visits either since it is causing him to get irrationally angry at your son. How sad all round- such a cruel disease.

This just how Mil’s dementia started. She became argumentative, lost her social inhibitions, and obsessed over incidents from the past.

I’d definitely be suspicious that this is dementia, especially with your context of him having always been a good man before and the fact it’s such weird outbursts.
what do you say when he starts verbally attacking your son?

Is your DH's sibling a sister and always been Grandads favourite golden child and now he's taking favourites with Grand Children?

I wouldn't be taking DS back, let DH visit on his own and if Grandad asks, tell him DS is scared of him and all his shouting.

My grandfather behaved similarly and did seem to pick out certain people for his behaviour. It was pretty strange as he was always such a lovely man. He took a bit of a dislike to me for a while and everyone was confused as we were very close. I could sense something wasn't right.

He was diagnosed with dementia and Alzheimer's a few years later.

I’d be worried he either has dementia or a brain tumour. Either way he’s a nasty bastard and no way in hell would I be taking my child round there anymore.
Why is no one sticking up for the lad? Neither you nor your husband are telling him to shut his mouth? Wtf??

Why the hell did you just sit there and not stand up for him and even bigger than that why have you taken him back. His behaviour is not ok. First time should you should jabe stuck up for him and after been told sorry GF mind not right and it isn’t ok and that you will talk to them. There shouldn’t have been a second time this happened, if there was I would have just stood up and left immediately.

Why are you continuing to put your son in these awful situations? If your husband wants to see him, he can go on his own.

I can't believe that you just sit there and say nothing while your FIL shouts at your young son! Why on earth wouldn't you say something and leave immediately? Stop exposing your child to this nastiness. Does your FIL have dementia? He sounds irrational.

Him"You need to grow up you are an adult now"
You"What are you talking about, he is 8"
"Him.... breaking things"
You"You seem very confused, what exactly has he broken?"
Him "Seeing how he is"
You "He is wonderful and has been rewarded for best in class, that is how he is"

Why are you letting this nasty old man bully DS?
Stop going, you are showing your son he is less important than an bully!

If I were there I'd probably say nothing at the first outburst. But I would have been ready for the others, I'd reply, stop being rude otherwise he's not going to want to visit you anymore. If he carried on regardless, then I'd stop visiting with my son, and send husband to visit his parents alone. If they asked where we were, husband would explain, they're not coming because dad is rude to my son. There had to be consequences for actions.

You say he's "started" which makes it sound like it's not always been that way.

Is it out of character for him? Does he behave in other ways differently to how he used to too?

If it is new for him, coupled with the behaviour you've noted and the decline in other areas I think his son needs to visit him, alone, and gently talk to him about approaching the doctor. It could be something as simple as a urine or chest infection making him confused or it could be something like dementia. Either way it needs following up on.

This. And stop taking your DS to see him. He doesn't deserve to be spoken to like that.

It is really damaging and you will need to address it or stop visiting.

He's a bully.
An adult bullying a child.
By standing by and not stepping in verbally or blocking contact you're enabling your child to be bullied.

My poor DGM went a bit like this. All her life she had loved children, bought them little toys and books, took part in a Volunteer reading support scheme in a school after she retired. As she became more unwell she changed completely and seemed to become resentful towards them , always thinking they were being noisy when they were playing perfectly nicely, and so on. She had no relationship with her great grandchildren because of this and they have no fond memories of her. It’s very sad. In her case it was dementia.

I don't say this lightly, but if your DH wants to visit his dad, he now needs to go alone. A weekly visit for your child who is his grandfather's verbal punchbag must be purgatory for him.

If your DH isn't prepared to have it out with your FIL, then he can't complain about you and your son staying home.

I agree that it might be dementia, but whatever the cause, the priority is to protect your son.

I have an uncle that used to do this. No dementia - he was just an arsehole. Turns out he was doing this to more than one of us, but never with someone else as a witness. Karmas got him now - he’s been cut off by everyone as they have reached adulthood. Serves him right.