Beauty is a very subjective thing, to be sure. Some folks cut on themselves (brands, tattoos, piercings) and regard it as some form of beauty. Some folks view big, or fit, or really skinny, the same way. Other cultures maim themselves in foot bindings / neck rings, etc. Other cultures don't think much of personal hygiene. Some folks even think being fat is o.k.
It is what it is.
The prescription for "big" is simple. If you workout, rest, and eat more calories than you consume, you'll get bigger.
At any rate, if you have some kind of limiting condition, you do the best you can to work with it. Without seeing the kid up close, it's hard to give good, responsible, advice.
These forums can be funny in that they often don't answer the question, but, instead, lavish affirmation at someone who may, or may not, need it.
Just FYI, more volume usually isn't the answer, especially if you're not chemically enhanced. Usually, the answer for getting bigger, is the typical plan for sarcoplasmic hypertrophy: moderate to high reps, calories, rest, repeat, patience.
Definition is a function of body fat levels. Particular exercises, other than HIIT, and steady state cardio, don't improve your definition very much. It's about the fat level.
Over the years, I've seen the smallest guys get the biggest, and, sometimes, the biggest guys get the "smallest." We often want what we don't have, and rise to the challenge of achieving a goal. The natural mesomorph doesn't want it as much as the littler guy, often times, and is content to do the bare minimum possible. The little guy wants it more. I don't think that makes him a head case; in fact, quite the opposite. (I'm a classic meso who also has a work ethic, so, it's a double whammy in my case.)
We do things not because they are easy, but, often, because they are hard. That's part of living the human experience for many of us.