You should get a litter box for him if you haven't already done that. They're pee is nasty!!!
Pretty sure ours was stolen. He was big enough to hop around out back at night, while the dogs were inside. When he managed to escape, he'd never really leave our yard. Anytime someone would walk up, he would run back under the gate. We usually go to the SPCA & grab the biggest rabbit they have. Definitely let me know if you can't find the owners!