You only mention it happening when you give him a bottle of expressed breast milk. For that reason, I'd have to say it sounds more like gas. If it was acid reflux, I would expect to hear you say it happens whenever he eats not just when you try the bottle.
Here are few things I would try... When my youngest was newborn, we tried using the First Years bottles. He had a hard time make a good latch on them so that he wasn't getting air with each suck. We switched him to the Avent bottles. If you haven't tried them, go and get an avent bottle. Playtex nursers have worked for other kids in our family too. Additionally, because he will get more air when using any bottle versus using a breast and will also get more liquid faster, you will need to stop and burp him a little more often. Until he gets used to the bottles and is able to better pace himself, try burping him after every 2 ounces (1 1/2 if he takes only 3 ounces at a time, up to every 3 ounces if he is taking 6 ounces at a time). If you talk to your doctor and he agrees that maybe it is gas and the other fixes don't work, you could ask him about using Milicon (sp?) drops.
Anyhow, I hope this helps. Good luck with your little guy.