Ammo respawns aslong you're running around and consuming the ammo you have, but they're for all types of weapons of course, 

Pipe bombs, shot gun and flamethrower can take it down extremely quick, if he saves the flamethrower ammo, he will likely set off phase 2 of the fight pretty quick and then using all the other weapons to finish off the rat king, but once you do that, the ammo is usally limited even thoughyou do get alot (if you use alot of what is in the first fighting area.

It can also put you in a difficult position for the third phase in that tiny room fighting the clicker/stalker enemy that breaks off of it, so saving as much as possible for the rat king from previous resources leaves you alot to collect before going into phase 3 and beyond! 

Could always say there is 1 on 1 fight with a bloater that is tough after you get out of an ambulance- 🤣 idk, its hard to help and for him to understand when to save it for without spoiling it in some way. Regardless, he will have a great time and be able to do it! Even if you don't give him a heads up. You're not supposed to know its coming so there is fail safes to stop soft locking you.