As a mother to a special needs daughter and as a woman who attends therapy for my own mental health, I'm in a unique position to post on this. It's a crushing blow to find out that there's even a possibility that your child is not typical and may have a disorder that could affect their life in a very negative and difficult way. Even though in the end, having a diagnosis is a good thing that can set you all on a path that will allow the family to heal and learn coping skills for what's been happening and what will come, it's very difficult to wrap your head around knowing, just knowing that your child has a neurological disorder. You can get therapies and services, but it won't fix the problem. And it's devastating, so it's easier to ignore it and pretend everything is perfect.
Then to add crappy icing to the crappy cake, he's being told that maybe he should be evaluated for the same disorder and marriage counseling would be a good idea. So he's not hearing, "Let's get help, babe. We can get help and make things right." He's hearing, "My son is broken, I'm broken, my marriage is broken, and I caused it. If I have bipolar/autism/whatever and she doesn't, but my son does, that means I caused it. Therefore neither of us has it."
So. Let your husband work out whatever he needs to work out in his head right now. Be supportive and let him know that when he's ready to talk, you're there. You do what you need to do for your little boy and yourself in regard to evaluations. Bipolar is very difficult to diagnose in young children and most doctors will be hesitant to do so because the medications are so serious, so don't be surprised if you don't get a diagnosis for that until he's older. Or he might get a diagnosis for something else.
Or your son might be fine neurologically and reacting (behavior wise) to the tension in the house and what's going on in your marriage.
I'm sorry for what you're going through.