I think your husband needs to have a serious talk with your son about his responsibilities as a parent. It's very likely his stepmother IS treating him badly, and making him feel unwanted, and your son is willing to go along with it because she's giving him sex and now they have a new child together.
Your son needs to step up and stick up for his son. I would be so sad to be in your position, and be so disappointed in my son for not putting his child above his own needs.
Yep, definitely some parenting issues. Perhaps order the book, "SOS Help for Parents," by Lynn Clark. It gives a good consistent system using timeouts. Give the book to them and tell them to give it a try.
Ah, this makes me sad to read. Here is what I fear . . . you are in the right, you love this child and he seems to respond to you, I absolutely agree with rockrose that the stepmom may not be so stellar and dad may be not either BUT . . . if you are vocal, they may cut you out altogether. That would be terrible for this boy! He needs you. These split families like this with young children can be so difficult on the kids. That inconsistency between households is so hard. Not eating could be as simple as things looking and tasting a bit different when step mom makes it! I have kids. They think I make the BEST turkey sandwich . . . um. Bread, turkey. Done. But when they go to someone else's house and have a turkey sandwich, they tell me it doesn't taste right. Who knows why . . . if it is the bread they have verses ours, the turkey or just loving being home with me. But this is how kid's minds work. Going between different houses causes this and with totally un- empathetic new step mom with a new baby who maybe views the boy as a pain? Ugh. She doesn't care about his feelings . . . just wants him to eat the dang sandwich.
You do what you can to stay in his life. I agree that a heart to heart would be fantastic for your husband to have with your son but be careful. You do not want new wife to decide that you and husband should not be around anymore.
Keep us up to date with what's going on! hugs (this has to hurt)