You can let him know that you've noticed he's gained more weight since you've met and ask him how he's feeling, if he's sad it could have more do to with his diet than him actually gaining the weight from just being lazy. But being lazy doesn't help either and or drinking.
Don't say it, it's going to hurt him. You can encourage him to hit the gym with you instead. Say that you want to become fit and ask him to join you. Besides find out the reason why he's suddenly become fat.