A few things I would mention...I think catching is probably the single position on the field where a player can do the most work and improve the most on his own. It is a position that, even with private instruction, is going to require the player to put in a lot of time between lessons working on his own doing what he's been taught.
If he's going to really work hard on his own without the supervision of the instructor, then I think once every week or 10 days is fine, and even once every two weeks is probably fine, especially in season where he's probably getting worked pretty hard in practice and games. Also, a great time to really work on this stuff is while catching bullpens. Most catchers hate them and only look at them as tedious work, but that's the best and most readily available opportunity for a catcher to work on his catching skills.
I actually think catching DVDs would are very helpful and generally give you a lot of drills and solid work to be done. There was a thread recently about quality catching videos, and most will set you back about the cost of a single lesson. One even cheaper route would be to put together a little library of catching skills work that is publicly available for free on the internet. If you look for the info, you would be very surprised what you can find for zero cost.
