Please help!

My 6 week old son has been dealing with some serious gas/ reflux. He was first switched to Gentlease in the hospital which really did help with the gas. Well, the reflux has still been an issue. His doc had him try Similac Alimentum which didn't change anything and smelled awful so we went back to the Gentlease. He's been on it basically since birth. He currently takes 4 oz every 2-4 hours depending on sleep. Well the reflux is getting worse. Today the doc suggested Enfamil AR. The first bottle we made he screamed because it was too thick for the size 1 nipples and he was getting frustrated. So we switched to a size 2 which helped but yes drinking them Soo fast now. Also since we switched to the AR he screams after feeding and shows serious hunger signs. I don't understand. I thought the thicker formula would fill him up more, not leave him screaming. I don't want to overfeed him so I just have an extremely fussy baby. SOS!