Agreed that finding a good catching instructor is not an easy find. I don't know if there is anything out there for preparing for a showcase as a catcher. At the PG showcase defense evaluation, it is literally only 4-5 pitches to catch and throw down for the Pop time. If you go to a PG showcase page, you will find links to lots of great videos with showcase tips. There is a good one on catchers, and the scout nails it on what they are looking for on pop times. A kid may try to cheat his position but the scout is going to know who has the legitimate Pop times, and there is video showing what the kid did on the web page. As the one post above noted, the one thing you will truly get out of the PG showcase is a valid metric that coaches will believe and will give your son a real comparison of where he is versus other players in his class.
Beyond the showcase issue, there are a couple of "catching camps" offered at various places where over a day or two your son would learn a lot about framing, blocking, positioning for the throw, etc. As a lifelong baseball fan, I had no idea how much is involved in being a catcher until my son switched to a catcher his Freshman year. The Stanford Catcher/Pitcher camp offered during the Christmas Holidays was excellent--my son's catching improved significantly after that one. That may be too far for you, but do a search and there is bound to be similar options for you closer to home. Good luck on the journey.