White lie praise or the honest truth about a teen's weight?

Say you know this kid (teen), bright, a go getter, always put in 100%, however, he is 5ft 6in and 286lb +/-. He has aspirations of joining track and being a pole vaulter. As good as he is in the classroom he sucks at losing weight or getting in better shape (loves his food too much). Do you lie and tell him if he tries hard enough and has a serious enough work ethic he can make the team and be a pole vaulter, or do you tell him the honest truth, even if it hurts, that unless he get the pounds off no matter what he does he will be too fat to be a pole vaulter, or at least one who will never have any success?