He's looks about right for length. My yearlings are usually between 800-1000 grams. I'd put him at around that, maybe closer to the 800 gram mark. He looks a tad bit on the thin side I think kicking up his food intake would be a good idea. The average adult mouse is about 20-25 grams and a 1000 gram ball python should be eating between 100 -150 grams in a feeding. 4 may be a bit much to throw at him in one feeding but you may want to feed 3 mice, every 5 days or so if you still want to feed mice.