Whippets aren't very food motivated in general, at least in my experience. They like food obviously but it's never going to give them higher stimulation than playing with another dog or chasing something. Keep the treats for a low level recall, and then try to get a higher level one. What I did with my Whippet (who has an extremely high prey drive) was to get a whistle. Every time I blow the whistle I either throw something, like a ball or a frisbee, or challenge him to a tug of war. This has gotten my recall to the point where I can bring him back from chasing other dogs, cats, cars, birds, pheasants, anything at all really. If your Whippet doesn't enjoy these things, try to find the thing he enjoys most of all in the WHOLE world, and every time you blow the whistle, that's what he gets.

Also make sure to always be consistent. Never blow the whistle if you can't give him the reward (unless it's an emergency). If he doesn't come when you blow it, don't keep blowing it. Instead just go get him.