So far he's using 87 pts. With all quests I think that puts him at 76.
So he'll have to get to lvl 95 to max any skills he's already got 1 pt in:
Max firewall for another attack (probably don't need another fire attack skill though)
Max static for a large range to static alot of mobs at once
Max tele for low-mana teleporting
Max warmth for faster mana regen
If he gets to lvl 96 he can max a skill that has 0 pts in it now & has no prereqs:
Max hydra for another attack (don't know if it would be any better than firewall)
Max frozen armor for the defense bonus
Max enchant to give your merc an AR bonus (fire dmg won't be very good)
Those are probably the only viable options. However if he really wants to stretch he might be able to make an attempt at maxing frozen orb, with a 1pt cold mastery. However, frozen orb would only end up at skill lvl 17 (18 if u get cold mastery from tal orb instead of a hard point) even if he got to lvl 99, which is no easy task. If he goes to lvl 99 and/or has enough +skills, frozen orb might be strong enough to use against fire immunes (after static of course). Regardless of dmg the chill might be useful too.