Yeah, a 10-gallon is way too small for rats, especially males. The standard number is 2 cubic feet per rat. Tanks are also not as good as wire cages b/c they have less ventilation - rats are already prone to respiratory problems, locking the urine vapor in with them doesn't help. Plus, they can climb the sides of wire cages and you can hang things!
A lot of people here swear by
Martin's Cages. They are very reasonably priced for the size - you won't find cages this big for this cheap in any pet store. The smallest needed for two rats is the R-670, but it's always better to get a bigger one if you can afford it, especially if there's any chance you'll submit to GGMR! (Gotta Get More Rats syndrome) Also - always get a powder or pvc-coated cage - plain galvanized metal soaks up urine and stinks.
If you can't afford a new wire cage right now, you should at least look into a larger tank. Or some people make cages out of big rubbermaid bins, by making a lid for it out of hardware cloth or other screening. Again, they're not as good as a wire cage, but it'd be better to have him in a bigger tank-type cage until you can afford a new wire cage. Who knows, you might be able to give the old cage a good scrub-down in the shower and get it usable!
I'm assuming he's alone b/c his friends died. It would be a good idea, once you have a bigger cage, to get him some new buddies - rats really shouldn't be kept alone, they can develop behavioral problems.
Good luck!
