Apart from all the good advice you're giving him, the fact that he is underage opens us all up to media crap when he or his friend has an "accident" and his parents blame airsoft for it.
So, are we endorsing a whole field of underage airsofters because 'it's ok, at least we're not selling to or playing with them'?
People that are underage have trouble resisting the temptation to bring the guns to school. To bring them out in public. To use them to vandalize, etc. (more so than so-called "responsible adults" who WILL get their ass kicked by the law if they do).
I don't think we should support underage airsofting. Period. It will eventually kill the sport.
Visit the board. Wait until you're 18. Plan. Be responsible. Don't figure out ways to buy guns and build trenches and alarm your teachers.