You said gassy, but it sounds like you're saying he needs to burp more UP rather than pass gas out the other end. Is that right? The reason I ask is that you'll do different things for each problem.
If he needs to burp more, I agree with trying different positions to burp. Also, you won't want to pat his back the entire time, but you can rub his back to help it come up. Keep at it longer than you think you need to!
If the problem is gas (as in farting), you can try gripe waters (Colic Calm is the best!), massaging his tummy and bicycling his legs, Mylicon drops, and eliminating some foods from your diet. While lots of people will tell you to avoid foods that make you gassy (broccoli, beans, cabbage, etc.), those foods can't actually make the baby gassy. Those foods make you gassy because they contain large starch molecules that can't be digested. Since they can't be digested, they cannot get into your blood and therefore into your breastmilk! here is a link that describes this: http://www.kellymom.com/nutrition/mom/mom-foods.html#gass... (btw, kellymom.com is the best resource for breastfeeding help/info!) However, if the baby is sensitive to dairy, nuts, soy or something else, that can cause excess gas in the baby, so you could try eliminating those things from your diet.
Hope that helps!