I completely agree that it's something more going on. Especially since he can get destructive at times, like using a steak knife to destroy the headboard on his bed. Yeah, that bad. I have suggested a therapist but am told he already takes medication for ADHD. His mom constantly uses that diagnosis for all of his behaviors. Anyway, I'm just the stepmother so what do I know.
10 years old is too old to be throwing tantrums. Something else is going on it can't possibly be just because a baby is coming. Just my opinion not fact. I would ask is anything else going on. Ask questions like How was school.who do you sit by at lunch, how do you like your classmates?
Make sure your husband reassures him and does not yell or Hey mad. Sometimes kids need a time to talk with no judgment even if they say something that is not acceptable or correct. They just need time to vent, have questions answered and figure it out.
My recommendation is to have him see a therapist. I know it sounds drastic but usually in this type of situation it's more than just anxiety about a new baby. It's a whole new ball game when daddy is suddenly having a bay with someone who is not mommy. He may feel that his mother is being replaced and that means that he's being replaced as well. It's very tough on children of divorced parents when one parent starts another relationship and another family. I've worked in pediatrics for quite a long time and have seen this situation over and over again and therapy does work wonders for these children.
Have your husband take just him out for ice cream or something. Make sure he asks him how he is feeling about the baby coming? Have him ask why his grades are going down and why he is acting out. This way he gets alone time with his dad and can express his feelings to him safely.