dear parents,
my advice would be to:
1st have his wrists checked, he might be hypermobile and in that case he will certainly need protective gear during his rides
2. regulate his activities, not too often and not too long on the bike
3. give time to recuperate if his wrists do hurt

Yes there could be long-term negative effects as his wrist bones are not mature and might not develop as they should under all this repetitive strain...

Go to a good sports/paediatric physio and let them see his wrists as the best advice cannot be found online...
you need some-one with knowledge to physically check his wrists. You have a quality bike for him so give him some quality health professionals time now.