My 2022 was in a similar situation last year. He is a primary C, but also played IF and P prior to HS. He was fortunate enough to get called up to varsity late in the season, but it wasn't for his catching (they had a 3 yr. senior starter). Instead, he got the opportunity because he was able to play anywhere on the field and he can hit. As I've told him since he was in tee ball and is often stated in this forum, if you can hit, they will find a place for you, and that is definitely the case in HS so make sure your son is putting in the time at the cages.
Also, as a freshman trying to play varsity, be ready for some mental games/hazing from some of the older players and coaches (I don't mean physical hazing.) They will sometimes say or do things to see how the freshman will react under pressure, and how your son reacts will go a long way in how they see him as a team member. Tell him to be respectful to the older the players and coaches, but remain confident in is abilities and don't back down if challenged. (Note: My son was much better at dealing with this issue than I was as a parent. Fortunately, CABBAGEDAD talked me off the ledge a few times during the season.)
Finally, if he does play with the varsity and it is made of up mostly older boys, do what you can to help him be part of the team outside of practice/games. For example, if they meet after games for dinner, drive him to the restaurant and let him hang with the guys (you can drop him or sit in another part of the restaurant). It may be a hassle to do this until he can drive himself, but that off-field bonding time is very important for team chemistry. (Note: I'm not saying to let him go to junior/senior parties on his own as a freshman, but where its appropriate (gatherings with responsible parents present or in a public setting like a restaurant), letting him hang out with the older boys will help him feel like "one of the guys" when they are on the field and at school.)
Good luck to your son.