That's a tough one. If you can, let him float in a tub of water for as long and as much as he wants. It will keep the weight off his feet. You will need to make sure the water is changed often, as the poop can cause infection if his skin is open. When he is out of the tub, you can smear neosporin on his webs and make a shoe out of a thick pad of non stick gauze and tape them on his feet. You will need to remove and reapply after each bath. This will keep his feet clean. You can dissolve one or 2 aspirin in a gallon water for him to drink to control pain.
Of course if he is in obvious pain and suffering, you may have to consider putting him down.
I knew a duck who had frostbite, but luckily she recovered and just has chicken feet now.
Keep us posted!