If I were you I'd trash the 55. But that's just me. 55's are too skinny and tall for anything really in my opinion. Get a 30 or 20. In reality, if it is eating good it is fine!
It will be fine for a while, all you have to do is add a lot of hides in there with a good size water dish and it will be just fine.
Hopper mice? Seriously? I'd be surprised if your snake isn't underweight - can you clearly see where his spine is, or is he properly round? A snake should be eating something at LEAST as big around as the fattest part of his body (no more than 1.5x that), but if your snake is underweight that won't be a good measurement, so you'll have to guess. I'm pretty sure mine was eating adult mice by that size.
Why would you even try this?
A bp will never, ever eat a cricket. They will technically eat a variety of rodents or the occasional bird, but listen to me - DO NOT FEED YOUR BP A VARIED DIET. They like to randomly imprint on very specific prey items, and if you decide to toss in a parakeet or hamster or something and it imprints on that...good luck ever getting it to eat anything less expensive. Many bps will even refuse to eat a mouse or rat that's a different color from what they're used to.
If your snake is 1.5 feet long and has only been fed hopper mice, I really do expect it's underweight. Ball python *hatchlings* are often started on adult mice.
My friend has a BP that will only eat white mice/rats, if it has a tiny spot of color on it it will not eat it.
do not handle right after feeding. your snake may barf it up. wait a day or two before handling him. also, don't don't handle him until he's been eating regularly for 2 or 3 weeks. handling a new ball python too soon can result in stress, which leads to a snake that won't eat, and eventually, death.
You can try holding the feeding tub over the enclosure and let the snake come out on its own 1/2 hr after it ate.
That wasn't the question, though.
Well for one, it probably won't even want to eat that soon since it's going to barely be done digesting its last meal. I don't know who told you a 1.5 foot BP should be fed every five days in the first place, but it should be more like a week to ten days. There is no animal on earth that benefits from being overfed and obese - not people, not snakes, not anything.
You've already been told that in this very thread. "can you clearly see where his spine is, or is he properly round?" A BP should not be tent-shaped, for lack of a better descriptor. If you want to see a "good" example of an underweight ball python, go check out your local PetCo. They can usually be counted on to have some sickly wild-caughts with spines showing.