It's good he's going to the vet.

Although it sounds odd, having a smaller snake in a large tank can cause them stress. Think about it this way... in the wild, they will live in old rabbit holes and similair places. Very tight, close quarters. They instinctually prefer the same type of space in captivity. While it's not as fun for us to look at and see them, it's what is best for them. Having said that, I'd say the tank you have can still work for now. Maybe get a few more hides in there, some fake plants/vines would help also. Anything that will make it feel like a smaller, tighter space to the snake. That will help him feel more protected and relaxed. It's hard to tell the snake's size but from the picture, it looks to be maybe just a tad larger than mine. I would say it's prey could be around an adult mouse or maybe a rat pup. Hopefully the vet will give you a weight, and some feeding recommendations.