I wouldn't try to spray anything, but you can put a handful of fragrant herbs (like mint, basil or thyme), fresh or dried, cut in small pieces in the hay bag before closing it. It will flavor the hay a bit and it can help. Still, I've been buying the same brand of hay for my rabbits for years and there are some bags that they like and some that they refuse to eat or will eat only some of. I still fail to understand their logic, as they sometimes reject a perfectly good bag of green dust-free good-smelling hay. It's hard to tell without knowing exactly what you feed your rabbit, but maybe cut down the pellets - if your rabbit is more hungry he will be more amenable to eat his hay. If you feel like he really doesn't eat as much hay as he should (about the size of his body in hay everyday) you have to try something soon - the lack of hay (especially the long hard fibers) in his diet could lead to GI stasis problems (one of mine went into stasis just because she didn't like the new hay bag and, having 2 rabbits, I hadn't noticed that she barely ate from it - she recovered but she could have died).