I am not familiar with using desitin on interdigital cysts, but luckily we haven't had to deal with them. As was mentioned, I think Epsom salt soaks are common. Another member has mentioned using Vetericyn spray too, which I know has helped us a ton when dealing with hot spots.
Interdigital cysts are often caused by allergies, so you may want to try Benedryl too if you haven't already, 1mg per pound up to 3x a day.
I'd be afraid the dog would lick the paw. Desitin probably has zinc as well as a steroid in it which wouldn't be good for a dog to ingest. I've recently dealt with infected hair follicles on the paws and also use Epsom salts to soak them 2 or 3X a day until healed. I also spray colloidal silver on them if the rupture. It's safe for them to lick, it's antibacterial and it heals quickly. It's my new favorite skin care weapon.
Maybe try putting some socks over his feet? Especially if you are there to make sure he keeps them on? I have also seen people use socks with straps that go over the back to keep the socks on.