Can my fjord carry me?

I currently ride a 14.1 hh fjord gelding. He's out of Stine by Gjest so he's very stocky but I'm 235lbs and I don't want to harm him in any way at all. I'm trying my best to lose weight, but I'm only 15 and my mom and doctor don't think it's right to perscribe pills but I've barley lost 3lbs in the past 4 months. Do you have any advice for me in losing the weight? And do you think that I should reconsider my horse choice? Please help me :(