<Hugs> I have ongoing problems with the bottle as well! A couple of things - have you got a teat that matches your flow? sometimes if you have a fast flow the slow flow teats can be too frustrating, also have you tried giving it to him when he's sleepy, like if ever you have to wake from a nap get straight up and give it or if you're doing a cluster or dream feed. I've tried all different teats but have actually found the short old fashioned straight up & down ones the best, the bigger flexible ones were too easy for her to just push out of her mouth & gag on.
But overall I find distraction helps which I guess is why when they're sleepy often works because they're not focussing on it. I will sometimes rock her in a chair or carry her around, jig a bit, walk from room to room, sing etc and then she starts drinking without really noticing she is IYKWIM. Its like you need to distract their mind & then their body will suck automatically. He knows how because hes done it before its just like my little monkey - now that they're older they're showing their preference.
Unfortunately I think what works for some doesn't work for others, my DD1 took a bottle fine, DD2 has been an ongoing struggle & even now I can only get a few ounces into her with distraction but we're working on it!
I hope it improves for you, its a real pain! Hang in there