I'm so glad your son is ok. Personally, I would have a professional bike shop put it together. Unless you or your DH know exactly where each and every piece goes and how tight etc. , better be safe than sorry. Good luck!
I already told DH that, even though I didn't want him to think I doubted his ability, I would prefer having a pro install this thing! Assembling a tricycle or simple, no speed, 2 wheeler is one thing....assembling an 18 speed bike is a complete other!! I know it's one project I wouldn't want to tackle!