My dog is not trained and he clobbers me when I try to play.?

My dog is a lab/ bull mastiff and I'm 13 so I didn't train as well as I should have he knows how to sit and lay down but the stay and come commands aren't getting in his head anyone have a suggestion? He also needs lots of exercise but my parents are 53/48 so they can't do anything about it so I have to and I weigh 86.0 lbs and he sits on me and bites my arms when exactly will he grow out of this stage he is year and 9 months? Please help I have a lot of younger members in my family and I have to lock him up just for them to play and I think I do way to much. Please help me?