Don't jump to conclusions! Generally, if you want to become thinner you do aerobic exercises, not just lift weights. Thirst can also be a sign of eating the wrong foods, especially too many salty ones. If he's thirsty from eating salty foods and quenches this thirst with beverages that include caffeine then he will be urinating a lot. And, if those drinks are sweetened with sugar or corn syrup, or contain alcohol, then they are full of calories and will cause him to gain weight. His MD might recommend a visit to a dietitian or a nutritionist. I hope all is well.