Forgot to mention this, but because he's a bugbear, I believe he is using a large execution axe. So its 2 d6's. which makes the brutal 2 and vorpal effect much more effective, IMHO.
Oh, strike that, you have to fix it then. My previously proposed fix should work fine. It would amount to 4.5 extra damage per encounter. A modest boost, that is more than the bugbear with the oversize weapon needs.
And for the record, I don't believe oversize weapons were ever meant to be in the hands of PC's.