Increase in weight is one of the most common side effect of steroid
. It also causes short stature, but it will too early to say this as a side effect in your case.
As his hair has regrown 80%, I hope your doctor must had tapered the dose of steroid. Steroid should not be stopped abruptly. There are also some steroid sparing agents which can be used instead of steroid.
Now I would suggest you to consult your doctor to evaluate the other side effects of steroid like hypertension
, eye problem, increased blood sugar, bony problem etc. and to taper the dose of steroid further.