Sounds like the all-too common cases of neglect I see in my reptile rescue work. You're doing the right thing by helping the poor guy out. You should be commended for your efforts.
Some snakes are very hesitant in switching to f/t mice. My best results have come from letting them thaw out for a few hours on top of the cage to get the smell in the air. Then if the snake isn't going for it as soon as you put it in, try moving it around a little (with long tongs in the case of your boa). Many will switch over eventually, but there's never a guarantee.
Jesse
Forgotten Friend Reptile Sanctuary
