Well, you only have to consider the logical weaknesses of being a huge dude who has spent his whole life farming and fixing shit, he has an abundance of practical skills but would have next to no intellectual knowledge. His size could also be his weakness, though you haven't decided to go down the 'bull in a china shop' clumsy route, he'd still be pretty heavy and lack agility. 

He could also have an abundance of superstitions or other things commonly attributed to 'bumpkins' in fantasy, which would also work with the lack of schooling angle.