My son did the reverse. He started with the pill and switched to injection. The reason was that the pill form was not being properly absorbed due to his inflammation.
Another common reason people are switched from pill to injection is the side effects are more prominent with the pill, nausea, flu like feeling, aches, due to the pill going through the digestion process whereas the injection bypasses digestion. My son had none of the side effects when taking the pill so that didn't play a role in the switch for us.
I hope the switch goes smooth for your son.