Almost all guys nowadays use the word “geshmak”. It has nothing to do with being Yeshivish. The Yeshivishe guys say “geshmak”, and the non-Yeshivsh guys say “it’s freekin’ geshmak”.
I am in that world and know it well.
Just be very cautious. There is a lot of very scary stuff going on in the teenage world nowadays. Even in Yeshivos. So while trying to protect your child from words like “geshmak”, he might get exposed to many things you would have never dreamed of when you were a teenager. I cannot specify on this type of forum, but I can warn you. bIt’s better to send to a sheltered place like Philly, who also take English seriously, then to send to other places, even if your son might use words like “geshmak”.
I do not know anything about Chofetz Chaim. That might be an option. However, as mentioned above. it is a little cult-like. They have their own way of thinking and their own way of living. I’m not saying that’s a bad thing. Just different.