OK, from a pediatric RN/C. Manager position, etc, I feel motivated to tell you something you probably know very well, if you interact with other parents of special needs kids from other areas. Even within the same state, what you might have available for your child in one place may not be the same as in another place--particularly with regard to quality of services. I mean, even getting special needs care providers--and feeling good with those doing it for your son, well, this may be quite a challenge. You could get lucky. You might even find that things are better in the new place. But given your whole family situation--husband and special needs child-- you really have to do your homework, and even then it may be risky for both of them.
If your husband is in a strong position where here is, starting over could be risky for him. If your child has a good set up with the currently provided services, it could be risky for him to move. In the interim during the period the family is not together, well, that's a big time stressor. IMHO, wherever you have the most support for your special needs child--that is mom and dad teaming it together--and possibly with other family or close support--and hopefully a great system in place for him at school and with his current services, to me the answer would be to stay put.
If you want derm, just kick butt in MS and on your steps and clinical rotations. If you score high enough and have good input from your rotations and LORs, these may well be the bigger factors for getting into a derm residency. If you are stressed b/c things have not worked out to everyone's advantage due to the move, that certainly will affect your performance. Plus the bureaucracy and red tape for your special needs son may be crazy--it really depends. I'd do the legwork on that, talk to the teacher and those attending the IEP, research thoroughly the services and red tape for the area you would be planning to move, and really talk with your husband about how he feels in his current position as compared with starting over--and the variables associated with that. That's a lot to do and do well IMHO. Do the research and really listen to your family and the most "relevant" others before you decide to make the move.
The best to you and your family