I think the standard recommendation is a year. I doubt there would. be a big problem switching to adult food a bit early or late. But when you switch you want to do that gradually.
I'd have to look, and I'm not sure I have records for that late, but 19 pounds at 5 months seems about right. Hard to say though. Some have posted here with what I'd say are insanely low weights and they have done OK.
As for how much to feed him, the best way is to look, keeping in mind that at under a year it's probably better to err on the high than the low side. Cups are not such a reliable way to do it because some mixes have a lot more calories than others per unit volume.
Seems like everything is good. No doubt 1000% better than what they would have gotten in Africa.