We injected both MTX and Humira for my girls. My older one is still on MTX and Humira.
They are much older now, but they were both young teenagers when they were put on MTX and Humira. The Humira shot does hurt and they didn't like it but they did get used to it. For them, since their joints started to hurt by day 6-7 (they had weekly shots), they really did know they needed the shots to feel better. So it wasn't a huge fight.
It did cause some stress, but it wasn't like they agonized about it all day. They both would be anxious maybe 5 minutes before the shot and then they'd get over it almost immediately and continue watching TV and eating their treat (we did chocolate or cookies or cake or something good to eat after the shot). It really was not so bad and it was convenient.
Both did prefer Remicade, but if distance is a big issue, then Humira is VERY convenient.
The MTX shot did not hurt much at all according to them, so they never really thought about it. We did Humira during the week for both of them, and MTX on the weekends. That way, they didn't get two shots at once.
We iced before and after the shot for 15 minutes and they said that helped. Asking questions during the shot might also help distract him - open ended questions work best, we have found.
It's true that he may outgrow Humira - kids often do - but then he can go to Remicade and hopefully by then there will be more options.
I will say that my younger daughter is quickly running out of options and it is very stressful but I honestly don't know what we could have done differently. It's just the way it is.
Good luck!