Actually, the Bible does speak about this, both directly and in a more general way. And I hope you'll take what it says seriously, because even if your weight isn't causing you any health problems right now, eventually it will -- as every doctor will attest.
One reason the Bible warns us against obesity is that it's often a sign that food has become too important to us -- in other words, a sign of gluttony. The book of Proverbs says, "Do not join those who drink too much wine or gorge themselves on meat" (Proverbs 23:20). Proverbs also uses vivid language to warn against gluttony: "Put a knife to your throat if you are given to gluttony" (Proverbs 23:2). One of the characteristics of a person from whom God has withdrawn His blessing is that "his face is covered with fat and his waist bulges with flesh" (Job 15:27).
But the deeper reason we need to avoid too much weight is because it eventually damages our health. God gave our bodies to us, and He wants us to take care of them. The Bible says, "You are not your own; you were bought at a price. Therefore honor God with your bodies" (1 Corinthians 6:19-20).
Take your son's words seriously; he loves you, and he doesn't want to see you harm your health. But more than that, take God's Word seriously, and make a commitment to honor Christ by the way you live -- and eat.