Iolite
Depending on where the tumor is on the leg, you can put the castor oil on a large cotton square or old wash cloth and place on tumor, top with a piece of plastic (saran wrap or plastic bag, whichever is easier) and then wrap over with pet wrap. You can get it cheaper at equine shops online (it used to be called horse wrap). The goal being to keep the castor oil on the tumor for as many hours as possible.
I spend about $19 for a bag of 48 medallions and Willow (she weighs between 65-70 pounds) gets 6 medallions for dinner along with about 1/4 cup of cottage cheese and a cup of thawed frozen dark green leafy veggie like kale, mustard or turnip. So a bag of Nature's variety lasts about 8 days. I buy the canned wild salmon and mackerel at the grocery store and pay about $2 a can and she gets 1/2 can, 2 eggs (fertile organic) and 1/4 cup of cottage cheese in the morning.
Or you could feed him the barf diet from meat you get at the grocery store, chicken being the most economical, adding eggs, fish and some veggies to round out the diet along with a soup bone or one of the bones that Nature's Variety sells. Willow will eat those bones COMPLETELY!!
If you're not giving him cod liver oil, it would be a good idea. I forgot the iodine, Trapper is spot on.