As he's now a mature snake, and it's breeding season, it's entirely possible that he has gone off his food for this reason. He's wanting a partner to make babies with. :P
Don't offer him food too often, he will learn to eventually ignore the food as a part of a routine. give him a month to build up a good apptite then try again. Keep the mouse warm so it gives off a stronger scent encouraging the eating response. Mine went off his food for 5 months, he didn't loose any noticable weight and was very eager to eat when he decided he was hungry after all.

Good luck!

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