Hi! Chiming in here as a mom who sends kids to dad, not a stepparent...
I'd definitely suggest getting him a picture of mom
Sounds like something that can help him keep the connection might be appreciated.
Is there a chance that he's feeling a bit distraught by having two separate households? Seems like 3.5 (or an infant!) is such a young age to have such long placements away from mom/his attachment figure. Perhaps this is an expression of tension over that? Have you checked into a counselor for an evaluation?
I don't mean to step on toes at all, and hope you aren't offended by my post. As a parent trying to work out visitation for my own girls, I spent a lot of time reading and thinking about what the girls' needs might be. We went to overnights very gradually, and always had the policy that "If she cries and wants to come home, bring her home." The point of visitation w/ Dad was not to meet some sort of set time standard, but rather to have an enjoying time together. We both agreed that if she isn't enjoying herself she'll just grow to resent it. (this is all the perspective of my youngest, who was born after the separation. My oldest hasn't had any problems going to dad's, but she lived with him for more than 2.5 years. And I also think it helps for there to be two of them...for instance, before dd2's first overnight, dd1 had an overnight alone with her dad. The next morning dd2 was really wondering where her sister was. When I explained she was sleeping at dad's, and that dd2 would be going over there at night, she was eager to do so. But there dad lives far away, so visits are clusterd with many days/a few nights every 3-4 months).
Anyways, just thought that maybe, since this seems to reallly concern you as being a problem for him, that it might help to get some outside opinion as to how he's adjusted to it. Even if it's what he's been doing all his life, it still might be hard to get used to shifting so much. Just a thought.
Sounds like you're a wonderful stepmama!