Try defrosting properly and warming properly. That should make a big difference.
If he's eating every week and has some weight on him, I would try F/T the right way, and then if he refuses, try again in 1 week. Then if he refuses again, you can offer a live mouse a few days later
You should not be offering food two days in a row. If he refuses once, that can also be due to stress.
If you could weigh him that would be very helpful.
If he's 200G, I wouldn't go more than 2 weeks, for example, without getting him to eat, if possible. However, at 350-400G, I would say try 3-4 weeks in a row with F/T until he takes it.
How often do you handle him? Do you leave him alone the day before and the day of feeding? BP's can be shy and often will not eat if feeling stressed for any reason.
Finally, what tank size, type, what temps, and what is humidity? Another reason a snake may not take F/T is not ideal conditions. You say you are new to this, so let's make sure everything is dialed in.