Specialize only when you have to. Here is a link to an article on college player measurables. It gives a rough outline on what you have to hit to get recruited at each level. I think it is over generous. Even if you hit these numbers it takes extra.
https://www.ncsasports.org/bas...ecruiting-guidelines
If you are too slow, MI and CF are not for you. JETson focused on Catcher and 3B because he is quick but not fast. Because he can mash he's played 1B, 3B, RF, LF, P and C. He has been recruited as a catcher.
Your skills/speed determine where you can play and where you can't. If you can hit, emphasis on hit, and field, you'll find a place on the field.
Having options can get you on better travel teams. They can have one of you to fill in a lot of back up positions. Catcher being a major need in many organizations. Then you have to prove yourself with the bat.
Saying that, only 12 players on a 35 man college roster are position players. There's more spots if you can sling it.
There is no right answer. Don't focus on putting a lot of effort into what you can't or won't do. Not too many lefty third basemen or slow short stops.
Focus on the real opportunities.
As far as weight gain, I recommend using the scale. He has to eat enough to gain 1 lb a week. If not, eat more. Pick a weigh in day every week.
Milkshakes with whey adds a bunch of really tasty, good, protein, sugar, fat and calories. Chocolate, berries, whatever you want to add. Look what milk does for the cows.