The clear belly definitely matters. It indicates either diffused, stripe, or both. In this case, diffused. And the head looks like masque to me.
The first thing I'd try to feed would be a boiled pink. See my threads OMG Three Babies Ate!!! and Diary of a Non-Feeder for specific cooking instructions and the most successful tricks I've used this season. Basically, thaw the pink in warm water, then pour boiling water over it and feed, still boiling hot.
I'd feed in a deli cup, in the evening, with the cup covered up with a towel or whatever so the baby can't see out. I think the tiny dark feeding container is nearly as important as what you are offering. Leave him covered for an hour or two without peeking.
If he doesn't take a boiled pink, the next thing I'd try is Ivory soap washed. It has to be Ivory original scent dish liquid. Snakes _do_ discriminate between soap brands, and Ivory is the most popular. But if he doesn't like Ivory, he may like Dawn or Palmolive or Joy or Dial. All those have worked.