looks like frostbite..and might have some infection going on..
might try some antibiotic ointment..tho he might need a systemic as well..from a vet for treating staph or strept..
another ointment that might be good is Panalog..(Animax is the generic)
do the feet have an odor?
poor little guy...
confine him to a small very clean area to restrict movement.
clean feet with betadine tea..keep clean.
give aspirin for pain and help blood flow..5 (325mg) tablets crushed into 1 gallon of water.
give electrolytes
give extra protein
apply antibiotic ointment
get suitable systemic antibiotic and further advice from vet.
may need skin debridement to restrict spread of possible infection and healing.
I agree, looks like frostbite. I'm sorry, poor guy. I wish I could help but I have no idea what to do for frostbite when it's at such an advanced stage.