what about thickening the milk, did you do it and did it help?
Well, this is something you can try, but i'd try the other things first. Usually thickening the milk is something you would do after advice from someone seeing you in person. If it's needed it can make a fair bit of difference as it also slows the flow down. You could try an Anti reflux formula- we have it here, it's mildly thicker and you can use it with reflux babies to help it stay in their tummy. It's also a little slower for them out of the bottle so easier for them to handle in their mouth... but you have to make sure the teat is suitable for it so he doesn't have to work too hard to get it out!
a slow flow teat may be ok for him though as he can probably control the flow easier out of it.. it's just trial and error!
See how he goes with other strategies other than thickening the fluid- he may be better as he develops more strength with age.
Again- there are a lot of symptoms that are the same or similar as with reflux, so it could be either- or a bit of both!