I don't see where a Service Dog would be of value to your son. Perhaps, you should consider an ESA, or just an all around good ol' family dog that is happy-go-lucky, patient, tolerant and likes kids, and maybe can do a few simple tricks your son can have him do. (catch, fetch, tug, beg, rollover, hide and seek, play dead, combat crawl... that kinda stuff.)
With your son's anger problems and the steroidal treatment with their problems... I'd be very watchful 100% of the time so the child and dog remain safe. I decompensate from steroid treatment, too.
Roxie