Aww, poor little guy! (I'm sure it's not easy for you, either, but you're 25, he's 8, and having a son myself, my heart goes out to him.)
I wonder.....do you think that his mom may have said something to him, or in front of him, about how you and his dad will probably have a baby soon? (Whether you're planning it yet or not doesn't matter.) Since he just started this behavior lately, I'm thinking this is a good possibility.
So in his little mind, not only are his mom and dad no longer together, and his dad married to someone else, but now there is going to be a BABY??!! A baby will replace him - he doesn't live with Daddy, but that baby will. And he's probably had lots of attention from you, but when that "dumb baby" comes, he/she will be your "real" child.
And maybe his mom did NOT say something, but the realization has come to him on his own. You've been married a year, it's a natural progression. If this is the case, no wonder he's upset!
And, even though he probably doesn't know exactly how it works yet, I'll bet he knows that when a man and woman love each other, they kiss and have babies. Most children think that kissing creates babies. At least it starts the ball rolling to what makes babies! :)
So, maybe you and your husband should sit down with him (or bring it up casually when you're out for ice cream, depending on his personality) and have a discussion about how SOMEday he will get a baby sister or brother, and how cool it will be, because he'll be an awesome big brother. BUT, you want him to know that HE will never lose his place in that family, and you both will love him the same or even more. And if he has any questions/concerns/thoughts/feelings that he wants to discuss with you guys about it, you are there to listen to him and explain things.
Blessings, J.