Paggy.
Life is complicated. And families are all meshed up together (or should be). Of course his leaving will affect the dynamic and the help with ds2, even if ds2 was fine and dandy everything would change. There is nothing wrong with being sad about change and worrying about how things will work, it doesn't make you selfish, just honest.
His being involved and part of the family and caring for ds2 helps make him the person you love, and the person that others love/will love.
And there are too many difficult decisions, should he wait? It's a risk, is it worth it etc etc? Not knowing what to think is a still valid with or without ds2 as a factor.
Start doing vague investigations into all sorts of walkers for ds2 now, just little initial looks for organisations etc. Arm yourself with knowledge. And action always makes one feel better (my mum says).