The 10-15% prey rule is for balls (and youngsters at that), not boas. At his age he should not be eating weekly. Many breeders start babies out on a 10 day feed schedule. He should be eating an appropriate size item every 2 ish weeks. My almost 2 year old boas are eating a medium-ish rat every 2 weeks. They are nice and lean, and I can see their muscles. He shouldn't be on rabbits yet. A male BCI, should be able to get by with rats his whole life. I would back him off. He is very large for his age, and imo, too big, and eating too much.