How do i tell my uncle he is pushing me too hard?

I been trying to loose weight for a couple of months now. I am very determine to loose weight. My uncle has been my trainer for the past few weeks. I don't mind working out with him. But at times he pushes me too hard. If i don't walk as fast as he does, he gets extremely mad. I tell him my cabs hurt really bad to walk a little slower. I understand no pain no game, but He tells me I'm making excuses but in a harsh way. I try my best and i put a lot of effort into it. I've eat healthy and excise. I don't have much experience in excising but i ales try. He tells me to do some excise with the ball but when i ask how am i suppose to do this? He gets really pissed off. I tried telling him he is pushing me too hard! I do admit i am overweight and i know he wants me to loose weight but he doesn't seem to understand because he was never been heavy. So if anyone can help me? it would really help.