I 2nd L.West's and Kinra's advice too!! What they said!

Try scenting the rats - rub lots of mice scent all over the rat right before you feed. Boas are usually garbage disposals so he should eventually go for it. You could also try just leaving the f/t rat in with him overnight (I would still scent it first). Make sure he's good and hungry first then get the rat nice and warm, scent it, and drop it in and leave it. It'll probably be gone by morning. We need to get him on rats soon - multiple mice won't do it for a full grown boa, lol!