They didn't really think through where that seat should be located.. directly in everyones pictures of the mountains was a huge fail... but as a heavier person, I know I'm heavy, I know I might not be able to ride.. I'm also taller and there's more than a few disney rides I need to get in very specific or I will not fit.. It's more in your head and how much of a big deal your family makes of the situation.. I'd rather know if I'm wasting 3-6 hours in line and get a clear view of the seat to know how to sit in it than not get in because it took too much time in the room.
but with that said, If he's tall and 300ish pounds, that makes him smaller than me and I can ride it and get locked in no problem. I suspect the issue might be how he has his feet positioned and it's a very confusing seat if your taller and to be honest, there is no actual reason they need to lock your feet into place except to force you to feel the banshee breathing.. This isn't going to help now but I have to lift my heals a little more for it to lock and then I can relax into position.
They have changed the seat since preview. The back restraint used to force everyone to lean forward.. It's been slightly altered to lock in further back now and some have mentioned how they seem to be able to sit more comfortable now..
The leg restraint makes no sense, it wont' be long until they just disable it. The back restraint is to make sure you can't jump out of your seat suddenly as your on a moving floor with a ledge in front of you.. The leg restraint, I can't think of any reason other than forcing you to feel the banshee breathing to have it.