First off, a 7 month old burm is probably too small to eat a rabbit.
You've got a couple of months yet. Maybe if you could find very small rabbits, or rabbit "pinks".
For switching over to frozen thawed the answer is patience.
Do not give up!
Start with maybe feeding your pre killed, then follow up with frozen thawed.
Once the feeding instinct is turned on, your burm should take the frozen thawed.
Most times, I just feed frozen thawed without a lead in.
I tease the snake using very long tongs, and usually they've always struck and fed.
In time I just place the prey item in the cage, and they eat at their leisure.
If they dont take the prey the first night...no food until the next feeding.
And so on. Burms can go a pretty long time without eating, as can most snakes.
Some can be pretty fussy over how their f/t is served.
I have one male that will only take microwaved rabbits or rats.
Something I am usually VERY CAREFUL doing. ( I have settings on mine that go from 10%-100% power )Others could care less how they're fed.
Be consistent, be patient, and you will be rewarded.