
I guess be sure it’s not intentional. Some people overeat to help themselves build muscle (bulking). One option is to identify the cause and suggest a solution without telling him he’s fat. Could it be his diet? You could say something like “I was thinking we could both start eating healthier together.”
If you want to tell him, you can let him know that you’re concerned about his weight gain. You can let him know it’s important to you that he works on losing weight (which might involve going to a doctor to determine it’s not due to some other issue than just the exercises he’s doing and his diet). But YNTA for caring about his appearance/weight if you make sure he knows you want to support him.