Don't know how young your stepson is, but I guess he is still quite young. Does he have a special room he can sleep in when he comes to you which he can regard as his own? Or a special bed with a favourite duvet cover on? If you can make him feel that one small part of your house is especially devoted to him and each time he comes, do something special with it, or put a little surprise under his pillow or a note which leads to a mini treasure hunt, perhaps it would give him something to look forward to. . Also, are there any small children around you of his age he could make friends with? If so, could you possibly plan a few outings or picnics with them when he visits to give him some feeling of "ownership" and continuity. Perhaps if you can create more of an environment where he feels he belongs he might not miss his mother so much and start to enjoy his visits with you.