Breastfeeding 36 week premature son

So my son was born 3 weeks early, all natural and he is my first. He is currently 12 days old and the latching to the nipple shield is getting 100% better, my worry right now is he was gaining weight and now he is losing again. We were one ounce from his birth weight just for it to decline again. He was tube feeding with the nipple shield by this last week every time I did it he wasn’t getting the milk it was falling out of the back of the shield and out his mouth so I quit using it.

I’m worried that maybe I need to go back to syringe feeding him with the tube but i don’t see the point when he isn’t getting the full syringe of milk that is given, and he’s getting milk without it anyways. Just don’t know how much.

Has anyone had this problem? Or any advise?