Wow, ok that brings the memories.
As for your problem, go back to a save game and see where exactly the problem occured. I seem to recall hearing about some kind of bug about stats not returning to normal after equiping certain cursed objects. Now mind you I could be wrong and am probably thinking about another game, but it seems likely that is the problem.
I have the silver edition manual right here and I'm not finding anything that would help other than the spells you just mentioned. It seems like he has some sort of curse on him but the remove curse isn't working. I'm not sure even a temple would help you if remove curse or disease didn't work. Energy Drain affects your life and you use restoration to counter the effects. So restoration wouldn't help. You could try getting him killed and resurrecting him to see if the stats return to normal.
Sorry I couldn't be of more help, but I never encountered that problem. Like I said, probably your best bet would be to reload an ealier save and keep a close eye on his stats to see if it happens again.