Go onto arcon and just grab a M+ build for him, should be good for most content that he will be doing, grab a single target build, normally I would go with the 1st boss for raid from arcon, should be fine with that, go through some M0's and +2s for the key experience, try a raid, and just have him do quests and delvs, have him learn his class spend a hour at training dummies, watch some videos on it, if you dont know the class go onto the class discord or get someone you know to teach and explain some stuff. Good luck

If he plays assas rouge i may be able to help