Figured someone would already give this suggestion, but didn't see it, so...
Have you tried pumping and then mixing 1/4 whole milk and 3/4 breast milk for a few days and then lower the ratio to 1/2 and 1/2, then 3/4 whole and 1/4 breast until he is only drinking the whole milk. You can also do the same with vanilla soy milk which is sweeter than whole cow's milk, but still gives the calcium and vitamin D and you can mix that with whole cow's milk to sweeten it up.
Also, you can actually "fool" your kiddo by mixing 1/4-1/2 chocolate milk with 3/4-1/2 whole cows milk. I do that with my son all the time so he is still getting the whole white milk, but it still tastes just like chocolate!
The other thing is that if you are finding that he is not tolerating the whole cows milk and soy milk isn't an option, we have something called Dairy-Ease, which you can find at Walmart or pretty much any grocery store, which is cows milk with the lactase enzyme in it to break down the lactose. It tastes JUST like regular cows milk, but is great for those with lactose intolerance!
Good luck!
S., 25, mom to Tyrrone, 1, newly weaned to fun sippy cups and straw cups (favorite are Nuby brand because they can go in the dishwasher!!!)