You say the meds are working, I assume you mean for underlying condition... So it's not like his life is over... Just have to adjust to a life long condition... I know it's not the same in any real way but there are other conditions that are life long so I would try that spin on it..
The problem I see is going to be the mother, sounds like she is going to poke her noses into this and the kid is not likely to want to share the news with her... Of course no idea of the kids age but hopefully they are 'adult' enough to be allowed to not be forced to share medical details with her.
Also I know we can't know much more, of course, but if you need to vent about it I am sure there will be many a shoulder here for you.