That looks fairly typical for a "baby snake". (And human babies have NO room to brag either- just saying.)
And I've seen similar stuff plenty of times from young snakes.
Another thing to consider is that in his new home (with you) he's probably drinking water that's different from where he was housed previously- & if you're using municipal water supplies, there are generally some purification chemicals added to it. We get quite used to it, where we hardly notice it, but a little snake that's suddenly drinking water with a little more chlorine (etc) in it might well get an upset stomach. Especially since his prey isn't doing anything to counter any reaction he might have to the change in water.
Water that's perfectly safe for the human immune system (of any age) may contain minor contaminants that will bother your snake just a little bit too. I used to live in the desert, & in the summertime the water coming thru the pipes became a bit warm, even when you turned on the "cold" tap. When water sits in the pipes & it's warmer than usual, it may allow some minor bacterial contaminants to proliferate just a bit: I don't believe it was a coincidence that twice during the summertime, the mice I raised happened to come down with diarrhea, & had to be treated with doxycycline (which rendered those treated unsuitable as feeders, but kept my colony alive for the future generations that were fine). I happened to hear that others in the area that also raised rodents noticed the same problem during the summer- that's when it dawned on me that rodents don't have the same immune system that we do. So that's yet another possibility- not knowing where you live, of course.