@mamag I really cannot answer your question, but I will be three years post-liver-transplant in September and the dose of prednisone is fairly minimal now, as I suspect your son’s will be eventually also.
I don’t think any pharmaceuticals are without some effects, long term if you have to take it for a long time or permanently. Frankly I take other drugs that concern me more than prednisone does, but that could be due to unawareness of effects that you are worried about.
JK