There is no set weight for age, honestly. Corn snakes grow mostly based on how much and how often they're fed. Their individual genetics also play a role. That being said, 122 grams sounds fine for a yearling. If you look at his body shape, you can tell if he's a good weight. He should not be triangular or have his spine showing. At his age, it's extremely unlikely that he could be fat, but if he was, you would see stretched out scales and fattiness right before the tail. Most baby corn snakes hatch out mid to late summer, so you can sort of guess when your baby might have hatched.