Its so horrid to see them in pain. I used frozen breast milk for Kaydi all she would have was milk so i made little ice pops from the milk. (Not sure if you could do it with formula.) perhaps icy water/juice.
We have an amber teething necklace too which seemed to help, but not an instant solution.
You have tried everything else i could sugest.
Lots of cuddles, let him know you understand.
CLaire