It'll be exactly like any other boy going through puberty--acne if he's prone to it, some squeakiness/cracking as his voice changes, etc. If he stops testosterone, the acne should go away and (I think though I'm not certain) that the squeakiness/cracking should go away as his voice settles as well. If he doesn't stop testosterone, it'll still go away in time, just like in cis boys.
He should definitely talk to his doctor, in any case. I'm not sure what doctors in your area are like, but mine never mentioned a timeline--it was clear to him that I was knowledgeable about what treatment would be like, I understood what would and would not be reversible should I choose to stop treatment, and I was eager to get started. This was good enough for him, and it should be good enough for any doctor that does informed consent.