I think that as long as it is what HE wants, and he isn't in pain from it (or complaining about it) - being active can never be a bad thing.
This is the classic "European bike racer" advice. Nothing is too much as long as it's coming from
him, and not some outside pressure.
"European bike racer" advice also holds that he shouldn't even have the
option of being a gear masher at this age. They (and I) would have you remove the big ring entirely and either replace the cogset or set the derailleur to limit him to no more than perhaps the 16 cog.
HTH!