Basically your answer is yes its possible to complete hell with pure fohadin, but you will likely feel severely limited in areas that aren't all undead/demon. But yes it's a nice easy fun build to play. Make hybrid magedin build to deal with immunitys perhaps or deck his merc out with gg gear to help.

Foh is kinda boring, and it is surprisingly limiting. Doesn’t hit beasts makes a lot of content difficult with no way to overcome it other than sitting there while the merc chips away, investing into other skills and gear to use a hybrid smite/zeal build, or skip it. There’s quite a few beasts in the game, from spike rats, to cows, to enslaved.

Hopefully your 10 year old doesn’t enjoy the idea of killing axe wielding cows.

I’d suggest Hammers, Poison Nova/CE, fireball/hydra, Java zon, or fire druid.

Some of those builds might struggle on Ubers and ancients, but can do the rest of the game in a breeze with a sunder and half-decent gear.

Or, are you making him build up on his own?