Yeah, undersized is a good descriptor for him.
It's true, gaining 50 pounds would not be a cake walk for him. Btw Dickie scientifically speaking your story about gaining 50 pounds when you turned 24 isn't because u grew up, You probably got fat. Maybe not visually obvious.
Puberty ends in late teens maybe goes until 19 or 20 in the late bloomers at best and lasts only a couple of years in boys. After that you don't magically gain 50 pounds. If you did, you have some serious metabolic issues. SOME people can gain a dozen or so pounds in their 20s as their metabolism slows, and then more into their 30s but what you're saying is nonsense. But 50+ pound weight gain within 5 years isn't "natural". I'd get checked by a doctor for a giant tumor growth somewhere in your body man.