Generally, by HS it's rare for a player to catch and pitch and do both effectively. Catcher's usually have a short throw for the pick off throw to 2nd (Ie, they don't bring their arm all the way back - usually throw from the ear). Catching is tough on the knees/legs due to squatting much of the time. Even if he doesn't pitch the same day as catching, the effect can still be felt.
If your son wants to catch, that's fine. He just needs to realize he may have to give up pitching. It's one of those times most players can't do both well.
HS coaches do like a player that can play multiple positions but if he wants to play college level ball (D1, D2, D3, NAIA, JuCo) they generally are looking up the middle of the field (catcher, pitcher, middle infield, & CF) and that they have the skill set and tools. I hardly call SS/2B a low impact position. Other than the pitcher and catcher, middle infield is the next most active position(s).
FYI - my son played catcher, 3B, 1B in HS and was recruited as a 1B for JuCo. After two years of JuCo, he received a nice scholarship from a D2 to also play 1B. Just shows you don't have to play "up the middle" to get recruited for college ball. It helped that he had good skill sets (hitting, fielding, throwing).