Baby bottles

I don’t usually post on here, but I’m at wits end. My 6 month old has dropped the amount of formula he drinks on average by at least 8 ounces. I thought I was because he hadn’t been feeling well. Then I figured maybe he was teething. As I was feeding him his bottle before bed tonight, I noticed that every time he would release the bottle, I could hear air being released. So I am thinking it a bottle issue. We are already using the size 4 nipple with the advent anti colic bottles and I don’t think they make any bigger sizes. I would desperately appreciate any insight or suggestions y’all may have. We have pulled back on purées and solids and offer no other liquids besides formula.