I STRONGLY recommend getting a fecal done and flagellates diagnosed by a vet BEFORE you subject the poor little guy to the treatment. Because the treatment is pretty rough on their innards. You know how antibiotics kill all the good bugs along with the bad? That's how my vet described it.

(And the treatment did help with the appetite problems in my two snakes who had flagellates? But it was diarrhea and OH MY GOD THE STINK once a week for three months afterward.)

Anyway, yeah, he's on the slim side, but I wouldn't panic. A lot of bps fast their first or second winter. Usually they wait until they've hit 500 grams, but I have a pied male at 425 grams that only eats once every three weeks, and only hoppers. He's about as long as yours, or even longer, and he's producing sperm plugs. And he's otherwise healthy, but man, I wish he'd eat something larger than his head.

So get the fecal smear done. But if the smear comes back clean, try not to worry about you guy too much for a while yet? Take his weight every other week, maybe try offering smaller food for while. If you get very desperate, you can try live hoppers. And if he drops 30% of his weight, THEN you should start really worrying.