Do you normally feed him in a high chair? maybe you could try just putting the ice blocks or just ice cubes on his tray for him to play with.  Tinned peaches were a fav of my girls out of the fridge, nice and soft but still cool, cucumber slices (with the skin off while his mouth is so sore).  Maybe try crushing ice in a bowl, then it isn't so hard?

Sorry I can't think of anything else & what I have given you could get rather messy but good luck in finding something for your wee man.

 Emz, you poor thing, you must be exhausted. You must be really patient, to have suffered through a week of this already.

I was going to suggest ice too, but see that won't work. The only other thing I can think of is letting him play in a paddling pool with clean water in it - it might take his mind off his teeth and perhaps he might gulp some water down as a bonus? That sounds pathetic now I've put it in writing, but I wish I could help out in some way. Could someone in your family look after Jack for a while so you could get some sleep?