Agree 110% if you can not afford to take it to the vet in cases like this you need to find it a home where the new owner could be able to take it to the vet.
Why can't you feed it F/T? He will not eat it then you need to try smaller meals, and try P/K. Med rats are as big as you need, try a med and a small rat and that would be able to fill them up. You need to get him to the vet, and deal with the price it is going to cost.
