Step-parenting with baby on the way: Advice please!

My husband has a 3 1/2 year old son that stays with us on the weekends. We have been together for a year and his son refuses to acknowledge that I exist. I have tried to play with him and make him part of the family (I have 2 boys of my own) but to no avail. He gives me the meanest looks when I ask him something. It kind of hurts my feelings that he treats me this way because I have always been so good with kids. I've never had one not like me! 
But now we are having our own child and I've just started really getting worried that my stepson may try to hurt the baby out of jealousy because of the way he treats me. Well I said something to my husband tonight about how I felt. I also don't feel like he is doing anything to prepare his son for the coming baby which will only make it worse when we just come home with something else that will require Dad's attention. I tried to tell him that he could at least point to my belly and say "there's a baby in there, that's your little brother or sister" and try to make it exciting for him. But he keeps telling me his son wouldn't understand it anyway. I'm so frustrated and worried about having my new baby around my stepson. I hate feeling like this. Any helpful suggestions? (And please don't attack me. I already feel awful for even thinking these things.)