Viv - if you are choking him when he starts to gag just take him off and let your boobies run a bit. Have 2 terry nappies handy. Bugger expressing it will only make you make more milk which sounds like you don't need! I have had to do this a bit too and my middy gave me this advice. Worked a treat and the supply settled quickly and I now have soft boobies again
Not great for feeding when out but that is a it difficult until they settle anyway as milk just seems to fly everywhere LOL! Good Luck you can do it!