Serious problem with dog, help!?

My dog is old, but is full of energy, lately he has these big holes on his Left Front Leg, on at the armpit where you can stick part of your hand in, and a few more smaller ones down lower. And they are spewing out puss! He wouldn't eat at first, but now he is eating and drinking, we can't take him to the vet, is there any cure? Or do we have to put him to sleep :'(