Have u tried upgrading his teat to the next stage? Well, my boy who was a heavy drinker started refusing milk too when he was at 5months too. Changed his teats, gv lesser intake etc. I got worried. But happened to be a phase. Guess they are learning to control their intake cus they want to roll over easier if they feel lighter and wants to learn to crawl.
https://sg.theasianparent.com/trouble-introducing-bottle-breastfed-baby-tips