Seriously, I'd just get on with the search. You can ask him (or tell him) to pay the rent for June if you can't find anyone, and see what happens. But if he says no, I'd just let it go. Having roomates brings this kind of thing up, it's part of the deal. Going to small claims court is just a hassle and an expense you don't need.
Can't imagine it would take too long to find someone else, would it? Even if you could bring someone in in over month, they could pay their part of the rent for however long they are there in June. How long would you expect it take to find someone?