Weeelll.... I have had a lamb lap from a bowl...

Try making a few changes so that if this is now a habit, it feels different and you might get a different response. A different bottle and/or teat, milk slightly warmer or slightly colder - or even switch to PSF for a feed or two, so it's a different fluid too; hold him differently, get someone different to try to bottle feed him if you can, feed him in a different place, different times.
If he continues his hunger strike, then I would, starting one morning, remove
all other food and water, offer him a bottle for a few minutes, if he doesn't drink, leave him (still without any other food or fluid) and try again 4 hours later. Repeat until he breaks. Increase the between-attempts time to 6 hours if he is particularly obstinate.
Yes it sounds brutal but you are trying to save his life. They can cling on with wisps of hay and little laps of water for a surprisingly long time, they usually capitulate quite quickly if those options aren't available.
If he doesn't give in on the first day and you don't want to leave him with no fluid overnight, either tube a subsistence ration into him (so he won't die but will still be hungry in the morning), or leave him some water with some milk in it.

I had quite a few led me this merry dance last year, and at least three where I thought I would never get them to drink. I did lose one of them, but only that one. Both the other hard nuts gave in when they had only one other option. Most of the others only needed one 4 or 6 hour fast to come around.
Good luck