I am sorry you are having such a rough time with feeds for your lo, and even more having such a hard time getting the doctors to listen.
A lot of time, I find is that the DR's do not want to say much about Reflux etc, as they do not know much about it. I really hope that you get some answers from the new doctor.
Emma has given you some fantastic advice, and it is definitely worth getting your lo's tongue checked to rule out a tongue tie.
Also with regards to the zantac, I am sure that this meds dosage goes according to your lo's weight, so your lo may need it adjusting due to having putting on weight.
With regards to teething, yes this could definitely be the reason why your lo is a lot more fussy. When they teeth, apparently this can cause their reflux to flare up.
Feeding your lo laying flat in the crib, this is not advisable, as it can be a chocking hazard, and can also cause ear infections, as it can cause fluid to be trapped in the ear canal. I know you say that it is easier to feed your lo when he is sleepy, but perhaps, you could prop him on a cushion to ensure that the is not completely flat.
Please keep us posted on how things are going. And yes, I have to agree with Emma, your lo is really cute.