It's interesting you say he chose to behave for your dh until he felt comfortable with him - this is more likely to be that he is behaving badly with those he trusts, those he feels safe with.
A GP is unlikely to know much about ASD, PDA, child's MH.
IMO you need to start researching, look into high functioning autism, PDA (pathological demand avoidance), different parenting strategies - with many children bog standard techniques just don't cut it. A couple of us have mentioned The Explosive Child - have you managed to have a look?
Unfortunately, if your child has behavioural problems, unless you are a crap parent (and I don't believe you are, as you are posting for help) you have to find out what's going on yourself. We've found that CAMHS and NHS services will see very obvious cases, but anyone falling under the radar will be dismissed and you may be dismissed as an ineffective parent, when it's probably not true.
So much of what you post sounds like my ds, who has HFA/PDA. He has no learning difficulties, he is bright and eloquent, he doesn't come across as autistic, unless you know him very, very well.
I also agree with Maryz. If you leave him behind, given all the difficulties he has, I think this could be a massive knock, and he may never forgive you.
I think you owe it to him to try to work out why he is feeling so bad that he behaves like this.