What I would suggest you do since he's currently under a vet's care is give the vet a call first thing Monday morning. You may need to have a repeat fecal done to make sure that whatever treatment the vet did actually got the internal parasites dealt with completely. A follow-up fecal would be my first thought.

Since the snake is refusing to eat and you see visible weight loss I'd suggest two things. One is to see that vet and have the snake reassessed. Two would be to purchase a proper digital gram scale to be able to track weight loss/gain properly. Eyeballing it unless you are very experienced isn't a good idea with a sick little snake.

Speak to your vet about assist feeding options. The vet may likely make that call based on his/her current hands on assessment and further tests on this snake. The problem with your vet doing the assist feed is you then have to take an ill snake, whose just been assist fed on a car trip back home. I'd hate to see what a regurge might do to an already ill snake so talk to your vet about that (you may have to leave the snake there for 24-48 hours to allow it to digest undisturbed rather than transport it immediately back home).