Have you tasted your pumped breast milk that you're offering your son through the bottle?
Some women have high lipase levels in their breast milk which can cause their breast milk to "turn" after it's been expressed. Lipase is a naturally occurring enzyme, but some women produce more than others. If you have high levels your milk would taste fine/normal coming out of your breast, but would likely taste really yucky about 4-6 hours after it's been expressed, even if you've refrigerated and stored it properly. The high lipase changes the taste of the breast milk once it's been expressed, causing it to take on a very metallic and/or plastic unpleasant taste.
If this is the case, you can correct the problem by scalding the milk right after you express it, which takes away the yucky taste and might be enough to make it appetizing again for your son.
Here's a link to a kellymom page that contains info about high lipase levels: http://www.kellymom.com/bf/pumping/lipase-expressedmilk.html
That's just one possibility, and the problem is more common than you would think. If your milk tastes fine even after 4-6 hours after you've expressed it, then your son may reject the expressed breast milk as a sort of protest - he may really want the breast milk the way he usually gets it - from the breast. That way he gets all the secondary benefits of feeling the safety and warmth of mama when he nurses, and a whole different kind of sucking then what he does with a bottle, and the milk is the perfect temperature coming out of mama! If he has another option (formula) for bottle feeding, he may take it, possibly not because he actually prefers the taste, but because he wants the breast milk the way that feels right to him. Having breast milk out of a bottle may call attention to what he's missing when he has it in a bottle instead of from the breast.
No matter what the reason, good for you for really trying to keep giving him breast milk! It offers so many benefits that formula doesn't, so I would try to prioritize the breast milk if at all possible!