I have smaller hands and find the trigger pull n some thumbhole stocks is just too long to be comfortable. I wish there was a muzzleloader out there stocked like my Tikka master sporter. That is by far the most comfortable stock I have ever shot.not sure what exactly bothers your brother about the thumhole but I'll tell you IMHO that it was one of the most comfortable guns to shoot because of it.
I have had friends shoot it as well and they all really like that feature.
I have big hands - maybe handsize has something to do with making it more or less comfortable?
Got to BPS and try it out for yourself if you havent already.