Is there anychance of his sons getting into racing? If so I would stay away from the TRX90. It only has single a arms, there are other models that have dual a arm setup that would be better in the long run... IF THEY ARE RACING. If they will just be rec riding the TRX90 is fine and the dual a arm setup is a waste of money.
Just remember that the TRX 90 lacks a reverse so you need to inform the kids not to get into a situation that they have to back-up or they will have to get off and push.
My son, who now has a Recon, learned real quick about this although he did push his TRX 90 backwards a few times.
The 90 is now regulated to the grandkids who love the thing!