LO keeps unlatching

I'm just curious if this is normal. I BF and FF my LO due to low supply. I do try to nurse as much as possible (some days I nurse every feeding then give a bottle of pumped breast milk or formula depending on how much I have pumped). My LO latches pretty well now after many issues and visits to the LC early on, but lately he keeps unlatching even though I know the breast is not empty (I can manually express it). Sometimes I pop him back on up to 4-5 times before I finally give up and move on to the other breast or just give him the bottle. My thoughts are maybe the milk isn't coming out fast enough for him or it's not enough and he's getting frustrated. I know sometimes it's gas that makes him squirmy and unlatch (grunting, farting, etc). Do any of you have any thoughts or ideas how I can keep him from pulling off all the time. Not only is it frustrating and time consuming, sometimes it hurts! TIA for any advice/help you can offer. Even though my supply is low I'd like to nurse for a couple more months at least.