Hi, Lora.
The fish-eating behavior of boulengeri is very interesting. We know they come from the subtropical rainforests of North Vietnam, and that they probably hang around streams and eat fish, especially when young. I would be willing to bet they're sorta like E. milksnakes that switch to rodents when older and can handle it better. Also, they may be a little like rough green snakes and hang around in shrubbery alongside streams, since they seem to be semi-arboreal. I'm thinking the appendage on the snout might be an adaptation for fish foraging as they put their head in amongst rocks looking for them. There's a water snake in South Vietnam that has this appendage and stays in the water all the time. I've also had ideas about them being somewhat related to the Gonyosoma sps.
I don't know why there hasn't been more written about their natural history and the behaviors in captivity. Of course, it was not one of the ones included in Schulz' book. Fell through the cracks, so-to-speak. Hopefully, those who have the sp. will keep us posted on these weird (for a ratsnake) fish eating behaviors.
Here's a trick you might use to get him to switch that I use with Butler's garters. Freeze a few pinks in with a fish and some juices in a plastic ziploc bag. The pinks will smell a lot like fish. Feed fish and pinks for awhile, then switch to mice. Good luck, and hope my trick works for you.
TC