This is just opinion like all posts here. I am on son 5 now and have always had them in grade equivalent once they reach HS. My 2017 is 5'10 140 with a late august birthday, so size and strength wise your son should match up very well at 16U and like your son if 2017 started school today he would be a 2018... He will be playing 16U because it is the most effective way to provide competition that will stretch his mental and physical baseball abilities. Last year in "age comparable 14U" play he led the team in almost offensive category and my position is if you are the best player on a team you need to find a better team. This year he will have to fight for playing time and the opportunity to start against older stronger players but it will make him a better ballplayer...again just my experience but I have always found that when young men test themselves against better competition it is us as parents who struggle, not them. We need to guard our ego and desire to see them as "the best" while they rise to the occasion, work hard to improve and appreciate the opportunity to compete.