Mountain Bike Impact on 7 year olds wrists - repetitive strain?

Hi there,
I'm after a bit of advise - my 7 year old son (and 5 year old to a lesser extent at the moment) has had a few problems recently with sore wrists, particularly his left. He is a mad keen mountain biker and is riding hard trails built for adults which are often very rocky and rough - and until recently with no suspension as decent bikes that small just don't exist. Now he is a bit bigger we have forked out for a really good german made bike with full suspension - this has eased the problem and his wrists are rarely sore when he is riding a lot. However, his wrists crack and clunk a lot - mainly the left (he is left handed) - and I often catch him clenching his fist which results in the crunch/clunk/crack.
I won't stop him riding, he lives for it and is very good (if I may say so myself) but I am worried about the long term effects.
Someone has suggested wrist supports, but my worry would be this may help short term but in fact weaken the wrists - might strengthening exercises help long term?

I'd be grateful for any suggestions / advise.

Many thanks

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.