That's how I understood it. You're allowed two home consoles and two handhelds on your account to access downloaded PSN content.
EDIT: There might be an issue if you try to log into PSN for anything on both of them at the same time. Basically, you should turn off your Vita, then go online with your son's to download whatever games he wants to play, then put his in offline mode before turning yours back on. I think that works, but I haven't tried it myself, so I can't confirm.
Anyone here have two Vitas on the same account?