I don't know BG, but he doesn't sound like he's got much WIS (especially early) - maybe keep that stat low-ish. You can still be effective as a cleric w/ a crap spell attack / save. Then you can put more resources in CHA / ST / CON as fits his character and arc.

As an NPC, how optimized do you want him to be? At what levels do you expect him to participate? His power progression will be very slow, but he's an NPC so that's probably desirable (and the DM can always choose to gift him a belt of giant ST or other boon if he needs a bit of a boost).

I'd assume he's mostly story and secondarily a combat asset mechanically.