I agree with the others, offering once a week is plenty, a little longer is ok too.
If you want him to switch, it's better not to give him live again because they tend to want that again. Once I get them to take f/t, I don't offer live again unless months go by and weight is dropping which has rarely happened and I've converted many snakes. The good thing is he has taken f/t, so that is the hardest step. At this point, you just need to let him get hungry enough to take it again. 99% of the time they eventually get used to it and take them weekly. If you want him on f/t regularly you need to stop feeding live. I know it's hard because you want them to eat but you need to stay away from live if you want him to take f/t consistently.