Why don't you take the AP ammo from him? He won't have a choice other than to use JHP ammo.
Right, the best way to manage companions in F2 is to leave them no choice... or not have them at all. The AI is too dumb.
To rectify the situation to a degree, though, take a look at FO2tweaks https://github.com/BGforgeNet/fo2tweaks
(includes alternate AP/JHP system) and Party Orders https://github.com/BGforgeNet/fallout2-party-orders
(allows to give the companions orders, including ammo type switch). The second one is essential, really. I dread the memories of playing without it.
Thanks, That seems to be very useful. It's amazing that people are still today releasing tweaks for this game!