He does not seem overly skinny to me from that picture. If he's fairly rounded and not triangular, then he's ok. Some snakes get longer first, some get thicker first. My husband has a pinstripe female, and at 1200 grams, she's a full 7" shorter than any of my females of that size, so she looks ridiculously fat. But, she's been that way her whole life. She gains girth for a while, then she lengthens out some, then she gets girth again.
In contrast, my cinnamon female is long and skinny looking. She's a healthy 1100 grams, but she's at least 6" longer than my other females of similar size, so she seems too skinny. But she's nicely rounded and plump, so I know it's just her length playing tricks on my eyes.
You do want his food to make a little bulge in his belly. Think Thanksgiving dinner, not hot dog eating contest. You'd be amazed at what these guys can swallow.
Gale