Thank you for all your helpful advice.
After a day of very painful feeding yesterday, we met the lactation consultant this morning. She could see nothing wrong with my techniques or positioning but acknowledged that his latch was shallow. So we went to an oral surgeon who checked his mouth thoroughly and there is no tongue tie or any other obstruction. His gingiva (the part of his gum where his teeth will be) is sharper than normal but he said that's fine.
Since we stopped using the shield, baby only nurses for about 15/20 minutes total but is satisfied and doesn't look for the breast for almost two hours. He is also napping between feeds which he never did before.
My paranoid self is wondering how he can go from taking one side every two hours for a duration of 40 minutes with the shield to this. He seems satisfied but of course I'm worried he isn't getting enough. I leave him on one side until he gets really fussy and pounding me with his fists, burp him then feed from the other side until I can't wake him to suck any longer. I always start with the least productive side first.
I have decided not to weigh him for another week and just try to nurse, nurse, nurse. It hasn't been as painful today so that's great.
So thank you again for your help. Baby and I are very grateful.