Just ensure he has strong legs under him. At 12 the worse likely thing to happen is he gets the wind knocked out of him. 90sec rds are quick so most novices throw a lot of punches in that short time.
If you are looking to test your kids wind, throw him on a treadmill, have him sprint 3mins, 1min break to put his gloves on, then give him a good mitt session. Do each 3xs. The sprints need to be fast and when u do mitts focus on him keeping his non punching hand up.
If he can do that, u dont need to worry about stamina but of course dont forget it.
If you are looking to test your kids wind, throw him on a treadmill, have him sprint 3mins, 1min break to put his gloves on, then give him a good mitt session. Do each 3xs. The sprints need to be fast and when u do mitts focus on him keeping his non punching hand up.
If he can do that, u dont need to worry about stamina but of course dont forget it.
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