First of all, welcome to the forums hun. As you can already see, we're all more than happy to give you as much help and advice on your situation as you need

You obviously don't need me to point out that you've got quite a serious situation on your hands there. If your son is indeed suffering from
Gender Dysphoria then it's clearly going to be a source of frustration for them. Bear in mind though that ADHD/Autism is also something that can produce significant amounts of frustration, so that does muddy the waters somewhat.
As the others have rightly suggested, get your son in to see a Gender Therapist / Councillor as soon as you can. They will help bring out your son's true feelings regarding their gender. They will also help in getting the right treatment to ease (and hopefully ultimately eradicate) the dysphoria, should it turn out that's truly what they are feeling. That might mean simply therapy, it might mean various degrees of transition. There are many forms transition can take, and it's not just a case of "boy or girl" either. A therapist will help them to find out where they need to be, even if it turns out that where they need to be is right where they currently are, or the opposite end of the spectrum, or some shade of grey in between

Also, I'd love to just take a moment to commend you - you're clearly very loving and supportive parents who want to do what's best for your child. Many around here have not had that support from their families, and would give the world to have had parents like you

I just want you to know that your son (or daughter) will be very appreciative of you for being this way with them. People like you make the world a better place, don't ever forget that
♥︎*hugs*♥︎ 
As for a place to speak with parents of trans* children - I don't know any sites off the top of my head, but give me a minute and I'll see what I can dig up. Hang tight

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EDIT: Ok so first of all, you're obviously more than welcome to ask questions here at Susan's. The
Significant Others section is a resource for partners, friends and (most relevant here) family members of trans* people. It's not specific to trans* children though, so the information you seek may be a little scattered.
Next up, I found this:
http://www.transgenderchild.net/resources/support-for-trans-kids-north-east/It may be worth getting in touch with one of those support groups there (note the New York section in particular - there seems to be quite a few groups right on your doorstep, which is always a good thing

)
Finally, there's good old
mermaidsuk.org.uk/It's a UK based organisation, but if you get in contact with them via email, I'm pretty sure they will at least be able to point you in the right direction.
Hope I've been of assistance hun. All the love in the world to you and your family, and don't hesitate to ask us if you have any further questions
♥︎*hugs*♥︎