From what I've heard it can be difficult to switch adults but here's what you could try:
First and foremost, work on getting him switched over to Fresh Killed (F/K). Even if you can't get him gto switch to F/T, F/K is better than live. All but one of my adults eats F/K, I do worry about the other girl but she refuses anything that is not alive. Can't even trick her with stunned or twitching, just whacked rats 
Many methods to kill the rat (Yes I said RAT, an adult BRB should be on a diet of med to large rats. Two kill methods I prefer are:
For small/med, grasp by tail, pin neck with stiff rod (screwdriver, pen, tongs... anything will do) and I mean press the neck down fairly hard. Yank the tail up, away from head and forward over head. When done correctly, you'll hear/feel the spine give.
( I know, not exactly fun but necessary)
For larger rats (and works on small ones as well) grasp tail, let dangle with head towards you and feet away. Swing over to smack back of head on hard surface or even better neck on sharp corner. Problem with this method is too hard and they bleed gthrough the mouth/nose. Too soft and the rat is just stunned.
If/when you get the snake switched over the F/K then you can try F/T. Could even do a F/K followed by a F/T. I raise my wn feeders so have never actually done F/T but I read you need to warm it up first (place in bowl of hot water for 30 minutes?
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Thanks,
Dave "Rainbows-R-Us"
0.1 Wife
0.2 kids
4.12 Brazilian Rainbow Boa
1.0 Hypomelanistic BRB
0.1 Het for Hypomelanistic BRB
0.1 BCI "Elvira" normal from 1989
1.0 BCI albino / het-anery
0.1 BCI Hypo / het-albino
0.1 BCI Anery / het-albino
0.1 BCI Hypo (possible super)
1.0 BCI albino het stripe
1.0 BCI salmon hypo
0.1 BCI ghost
lots.lots.lots feeder mice and rats 