I had been through the mill and back with ds when he was that age too. It sounds like him to the T! I figured out it was a mix of a heavy let down, and getting too much foremilk, and no tenough hind milk, and also his temperment. I know wha your thinking, but ds really did have enormous amounts of stubborness and personality at that age. I had a very heavy letdown, and even if I expressed some at the beginning, it didn't matter, it was still squirting him in the back of the throat, and I'm sure that was uncomfortable.
I highly doubt it is an allergy related problem. It is very uncommon to find a baby who is allergic to this mothers milk, or somethin gshe is eating, but trying to cut the most obvious might be e good idea to see. Broccoli, asparagus (and other gassy veggies and foods) dairy, spicy foods etc. It's probably not the problem, but I would keep a diary for at least a week or two.
That said, ds had gas and uncomfortable BM's because of the inbalance of foremilk and hindmilk. i had to work on this my taking him off the breast after the letdown began, and waited a few moments, and put him back. It made him mad, but eventually my letdown became less strong(although it always remained a bit). Ds got used to it, and my body adjusted.
My biggest suggestion is to call a lactation consultant(your hospital can suggest one) or look on the LeLecheLeauge site can help.
Your littl eone is getting smarter, and knows what he wants, so don't be shocked that he knows how to show it. When I would get very upset about nursing given the stress we were having, he would feel it, so try to relax.Eventually he needed to nurse less often ,and he got so good at it, I only nursed for a few minutes at a time, and he was done. Keep working on it, breastfeeding was the most rewarding experience of my life. There were so many ups and downs, but at the end of the day, I still loved it. Take it one day at a time. I hope this helps a little bit!