My older brother (who was from our Mom's first marriage) used to tell me I'd been dropped on my head after I was born and that kids don't count until they are twelve years old (he was twelve at the time

). I remember feeling angry at him and I went to our Mom and she straightened me out. The point of this is to say that all older brothers seem to do this to little siblings. The son that is doing this is clearly not making fun, he is showing a problem that needs help. I think it was Green who said that this needs to be aired out with all family members present; furthermore, I think your hubby needs to do the majority of the talking, since it is his son that is doing it to the younger. I think your hubby needs to spend some good quality time with the older son, and give him some attention and affection. If this doesn't work, it's off to counseling, no time to lose.
PS I always thought of my older brother as all mine, that is, not a half- or step. There was not a whole lot of chat about "who was what" in the relationship department in my parent's house, and I think that was a very good thing.