- Does your son ride a lot? If not- that's most likely the actual answer to all this. Experience and practice builds muscle and ability.
- How steep is this 600' climb and how long is it? 600' of straight climb is nothing to scoff at, but if its 10mi long thats a lot different than if its 4mi long.
600' of straight climb though...thats tough. Its especially tough on gravel/dirt.
- If you change to 3x, then you need new shifters too. Also a new front derailleur and maybe a new rear derailleur(depending on what model is on there).
Not cheap.