He may give you permission to use, presuming it's for a non-$ purpose.
But the minute you start making $$$, he'll want a piece.
My advice? Buy out. If you expect to make say, 500 bucks from them, buy out the use for 200. One time fee.
If for whatever reason, he wants royalties or any other type of deffered comp in this type of scenario, walk away and use something else. Not worth the headache unless you really feel strongly about the use and the ability to earn the coin on it.