Hello and welcome to BW forums
The only things I can think of (other than asking your medical professional for advice) are:
- to start out with a syringe to put small amounts of milk in his mouth so he gets a taste for it and then to ensure the bottle teat has milk on it so he can taste it before he begins to suck. You must direct small amounts of fluid to the side of the mouth (aiming at the cheek) and not towards the back of the mouth/throat.
- to stimulate his sucking reflex by gently touching the roof of his mouth with a clean finger before introducing the bottle teat.
I hope you get some more ideas from others who are more experienced in this area.