Maybe he feels a little resentful of your new partner and his son, think of it from his point of view - he had you all to himself and now two new people have entered your lives, and he s now got to share you. Also maybe hes a bit worried he may not be the most important person in your life any more. A lot of reassurance and lettin him know he is the no.1 man in your life may be just what he needs to hear at moment. Remember hes just a kid and cant express his feelings the same way an adult would. Hope it all goes well for you all.