Corn oil is not a good fat source.
First, address his overall nutrition. He needs to be getting plenty of hay, preferably free choice grass or mixed grass hay, and a diet that balances his hay/pasture. I prefer to use a vitamin/mineral supplement with alfalfa pellets and flax, but there are pre-mix feeds that work well, like Triple Crown's 32% Supplement, Buckeye's Grow-n-Win, Progressive's Grass Balancer, etc. Feeds high in starch/sugars and byproducts are not good for growth or muscle development, like sweet feeds, Safe Choice, or any feed that has corn, sweepings, leavings, etc.
Once you have his diet figured out, you can add extra fat with flax meal (up to 4 cups a day), extra protein with Alfalfa pellets or cubes (3-6 lbs a day), and good amino aicds with a supplement like Uckele's Tri-Amino. You need the right combination of fat, protein, and amino acids to build muscle and improve health.
Also keep in mind that 2 is young. Young horses will go through growth spurts that make them look skinny or gangly. Just give him time, turnout, and proper nutrition, and he will develop in to a good looking horse. Most horses will continue to grow in hieght until 4-6 yrs old and will fill out from 5-7 yrs old.