I'm sure Kristin will be along shortly to answer the more knowledgable questions, but BB does make liners to help with the under arm areas for those that are sensitive. I think they are in white but those that do get irritated and wear the liners are perfectly fine.
I personally would get a 3 if he has a 29cm/11.5" chest. This gives him some room for a bit of growth, in case there is any.