I agree with the suggestions not to put the food in his mouth, just put a piece or two on the tray before you feed him and let him look at/play with it for a few minutes before you start feeding him the mushy stuff. Don't give him much, since it will mostly all end up on the floor eventually.
You might also try putting a favorite pureed food in a bowl in front of him for a few minutes before feeding him to give him the idea that he can feed himself (mostly by sucking the mess off his fingers, of course).
Softer, moist finger foods may appeal to him more as well. Try a chunk of banana, steamed-to-mushy broccoli, cooked peas, or adding a dash of milk to the cheerios (just enough to make them soft - not dissolving!)