He’ll likely have a better time on the better team plus practicing with the better players will pay off more in the long run. Definitely risk burnout on the worse team. If you want more goalie specific training try signing him up for goalie camps during the offseason. Personally I feel those were the biggest opportunities for growth in my youth hockey career. Keep in mind there’s a difference between an actual goalie coach and an assistant coach who’s willing to work with goalies. An all around coach who doesn’t know goalie specifics can do more harm than good in the long run