The size and age don't suprise me but hearing that he eats fine and is that small doesn't sound right. I have a male clown that I found on Craigslist that was 6 months and only 160g when I got him. He is now 11 months and only 190g. Husbandry is on a point and he's otherwise healthy but he is a major problem eater. He is strictly a mouse hopper eater. Won't take a rat pup, won't take anything larger then a mouse hopper, won't take f/t. He also will only eat for 3 weeks consecutively then he goes off for a month or so before eating for another 2-3 weeks.
If your snake is a good consistant eater then you might try feeding more often. Feed every 5 days til he gets to 500g