I found with my snakes, and usually the problem with others, is that when you switch from live to f/t, there are three big factors that make a ball python not switch:
1. Smell. Sometimes it just doesn't have a strong enough scent to trigger a feeding response. Scenting helps a lot, but sometimes even using poor bedding may not help. The first time I tried scenting, I went to petsmart/petco for some used bedding. It was like a soft, colorful, almost paper-y bedding. It didn't hold scent well, so it didn't work. The next time I went to a small, local exotic pet store. They used a pellet-like bedding that held scent VERY well (probably because it may have been older, less fresh, and/or much more live rats than a small few). It was very easy for me to smell, so when I used it to scent, it helped "soak" the smell a lot more than the other bedding. When you do it, it may also not help as much if you just rub the bedding on. What I had to do was heat up my thawed rat WITH the bedding for about 20min to get it to really smell.
2. Temperature. There are tons of ways to thaw your rat and each individual snake have different thresholds for triggering a feeding response. Some, usually adults who are used to eating f/t, will eat a rat at room temperature. For those switching from live to f/t, it's usually necessary to get the temperature to mimic a live rat. Typically the body temperature of a live one is about 100F, but I found--personally for my snakes--they took it at around 90F+ when they first switched.
3. Movement. This one, I feel, is a small aspect, but one that can help a lot. When I feed, using tongs of course, I find it really helped to wiggle the rat slightly to mimic live movements. Nothing too fast, but just small, short shakes or slow back/forth movements. This, coupled with the right temperature and smell, is often good enough to entice them to eat (given proper husbandry, namely correct temperatures for digestion and enough cover/hides to feel safe).
That being said, I mentioned that some methods work well for others and some don't. Just in case, if its your preference, this is the method I personally use: I put the rats in a ziploc bag and move them from the freezer to the fridge the night before I feed them the next day (so about 24hr to thaw). The next night, when I feed, I fill a cup of water from the sink and put it in the microwave for 50sec (just because my sink doesn't heat water fast). Then I put the ziploc bag with the rats in the cup and submerge them (usually with something on top to stop them from floating like a rubber duck, another cup, etc.) for 20 minutes. [Back when I was switching them from live to f/t, I put the used bedding in the ziploc bag with the rats before I submerged them in the cup of hot water.] After the 20 minutes, I just used tongs to feed them (grabbing them behind the shoulders instead of by the tail helps make it look a bi more natural, as well with a little "zombie dance" to make them look alive). Since you're working on switching them from live to f/t, it may be useful to check their temperature with a temperature gun before you feed. You might have to change out the water and heat it up again if it's not high enough. When I was first switching, I would also hit the rats (all over, but mostly head) with a blow dryer on low heat for about 5-10 seconds. If they get wet from the cup, it may be useful to also dry them (some snakes may not like wet prey).
I also fed live until 300g before I tried switching them over. You don't want to risk having an already thin snake going on feed. I stuck to the normal feeding schedule, but if they didn't eat 3 attempts in a row, I would feed a live one (you want them to switch, but not to starve them until they're hungry enough to eat!). Make sure not to move them from the enclosure as the movement can stress them out (as well as cause them to regurgitate after eating when you move them back into their normal enclosure). Feeding at night--since they're nocturnal--helps as well as no handling or bothering 24hr before feeding. Sometimes deep cleaning the enclosure two or so days before feeding can help. I've also seen some snakes that will react to a still rat suddenly being dropped (it's weird, but it worked once for mine, but he was already an established f/t eater). Double and triple check your temperature, humidity, and set up. Low temperatures can make them go off feed since they need the heat to properly and efficient digest. They also need enough hides/cover to feel safe enough to digest, so if it's too open, that can be a small factor that prevents them from eating. I've also even seen a few snakes that will only strike if they're really pestered, but I wouldn't recommend this off the bat since it can really stress your snake out into not eating even further.
The main thing to keep in mind is to just be patient. If he looses too much weight, don't worry about switching back to live. Keep in mind some snakes may never switch, and sometimes it can take many months. It took one of mine about 8-9 months to fully switch over, but the other switched right away.
-firefli