I am pregnant with my second child. My son is 13. His father and I are divorced; it was a bad marriage and my son was rejected by his father. Since the divorce, his father fades in and out of his ...
Respect his decision. Talk with him and tell him that you don't intend to exclude him, but there are going to be a number of things that revolve around the new baby as is normal when a child is born because they take up so much time and are helpless. Tell him he is always welcome to help and participate, but that you won't force it on him, and in respecting his announcement, will try not to approach him or ask much of him in assistance, but that you'd certainly appreciate any he might have to offer if he changes his mind. You'd love it if he would be more involved in the family as a whole, new baby included, but that you don't want to require it of him in respect for his stated position.
That being said, I think you have a problem being weak as a parent - please don't confuse 'weak' with 'bad' or 'incompetent'. It is the only reason I offer this advice. The fact that your son stepped up to say this, and that you allow your deadbeat ex in and out of his life so that he can continuously revisit the rejection by his dad says reams about your parenting style. He's obviously strong-willed, and has been allowed to be so by your parenting style. While this is not a bad thing in general, it is when at 13 he feels comfortable enough to disassociate himself from you and the rest of the household family - it is not something that most 13yo's would feel comfortable standing up and saying, even if they might actively pursue it quietly. Perhaps a stronger stance earlier on, or a gradual, progressively more strict approach now to handling the teen (and his errant father) might be warranted here, but the first position is to show respect to his proclamation, while simultaneously letting him know that it might have unexpected results causing a greater exclusion of him from the rest of the family.
However, you and your husband should also take special care to request one-on-one time with him in activities that he enjoys (batting cages, paintball, movies (as a last resort), etc). This will let him know that you keep actively trying to include him even while he excludes himself.