if he is 2 or 3 months old, he should be approximately 15 to 18 inches long and weigh 10 to 25 grams.
You would not be the first person a seller has lied to about age and feeding.
If your snake is less than 15 inches long, I'd speculate he's only a few days or weeks old. (Corn snakes are typically 9 to 12 inches long at hatch).
It's always best to wait and purchase a snake that is at least several weeks old and has fed a minimum of 2 times with no problems. Of course just because a seller tells you, "Yeah, he's 2 months old and he eats pinkies" doesn't make it so.
If you bought him from a seller that has a good reputation for raising and selling healthy baby snakes, then he will probably work with you on the problem. If he's "Not" willing to work with you on the problem, then I'd be willing to bet he either has no reputation at all, or he has a bad one. Either way, you have a snake that won't eat.
I'd double check everything. Temps @ 85 hi, 75 low, 2 hide boxes - 1 on each side, fresh water daily, no pine or cedar in the cage, and measure and weigh the snake.
Pinkies are for snakes at or below 15 grams. Above 15 grams you feed double pinkies, and above 24 grams you feed fuzzies.