Hi Beth,
I am not a garter snake expert, but I can give a few general tips until someone with more specific experience chips in:
Is there a female garter snake anywhere in your room that he may be smelling? He doesn't need to be right in the same container to know she's there.
If he isn't losing weight, don't worry, snakes in general can go off food with no ill effects for an amazingly long period of time - 3 months is not a surprise if it is cold or breeding season. I would keep weighing him regularly though - if he starts to lose ground it's time to do something.
If you do feel you want to kick start him, try a goldfish - I did use to keep a few garter snakes (many moons ago ) and nothing attracted them more than live goldfish swimming in their water bowl.... once he has started to eat again you can work on getting him back onto mice.
Hope that helps a bit - I wouldn't worry too much yet, chances are one day he'll just decide it's all over and take the mouse as normal.
There are a few old garter snake discussion threads here that might give you a few tips:
http://www.faunaclassifieds.com/foru...Feeding+garter
http://www.faunaclassifieds.com/foru...Feeding+garter