Have you tried picking your partner up on it when he does it? Probably not much use having a general conversation about it, because if he doesn't realise he's upsetting DS then he'll not know what he's been doing wrong. But you could try spelling it out for him whenever you think DS is confused - so he realises why DS is confused by his behaviour. As a parent you are just a whole lot more sensive to your child's personality and feelings. It's very easy to misjudge things as a stepparent. If you DP is willing to listen to you and try to change the things he does that upset DS, then he should learn in time.
Also worth having a chat with your DS after he's been confused to help him understand the way your DP is. Does DS maybe see him as a friend sometimes and then get confused when he acts like an adult who's in charge of him?
I know, there is no swearing or anything.. but it is just so confusing and of course my son finds it even more confusing. Luckily he talks about stuff what worries him and just asked me now what was going on yesterday and why my partner was like that, so I get the chance to explain it to him. he had a lot to adjust to, we moved country, a new language, his dad living still in the uk (he comes to visit a lot and my son goes back for holidays), new school, etc... No one can be perfect and do everything right but still...it's not on.
I think you're right to say he forgets your son is just a kid and he needs to be the adult.
If he doesn't have kids of his own, he probably just sees this little person who he can have a laugh and a joke with and forgets that an 8 year old I'd more sensitive than an adult friend would be.
At eight kids can be very chatty and funny even with strangers, but if you relax your guard and talk to them like you'd talk to another adult it can cause confusion at the least or hurt feelings and upset. I don't mean swearing or inappropriate jokes btw, just that kids and adults react differently to "banter "
I actually don't know how to describe it but at the moment I'm struggling with how my partner is with my child. We live together since a year and I'm sure it is not easy becoming some sort of 'parent' over night. He has got those mood swings and my son can't keep up with them. My son really bonded with him and they do have some great times, I think my partner feels some bride with being around my son and a little family. But then...on some days he just can't stop making it complicated for my son (8yrs). They both have their own boyish humor about stuff I would never laugh, non stop jokes on the table at dinner time, messing around but then, like today when my son starts to have fun with him my partner just gets cross and puts him down about why he would say such silly things now. It's very confusing for him. I have tried to talk with my partner about it but he doesn't get it. I actually think that he is forgetting sometimes that he is 43 and my son is 8 years old... I think he is confusing him a lot and it might harm this relationship because I can see that my son is loosing a bit of trust in him. He had to learn a new language too to talk with him, since we have moved back from UK to Germany 3 years ago and of course can't sense the 'between the lines' situation that fast. It's very frustrating at the moment because we actually wouldn't have any other issues together but it bothers me a lot and I see that there is a problem arising and I don't know yet how to deal with it.