Playing around with Wilson a bit, I've noticed that his strength increases every time he leaves and rejoins the party. If you recruit him at level 9, his natural 18/00 strength goes up to 19. Leave and rejoin, and it becomes 20. Again, and it's 21...
At higher levels, this goes even faster. If you recruit him in ToB and level to 18, that strength is 20. Leave and rejoin, 22.
Anyway, this problem has a simple cause; the timing mode on the ability that increases his strength is wrong, causing it to be recast every time he rejoins the party. Here's a quick hotfix for it; put the ohrbear6.spl file in your override folder.
This bug and fix is current as of game version 2.5. I'm not aware of any mods out there that address it.