Rats or mice would work, my snakes take either. I also use African Soft Fur rats, none of them really prefer one over the other. I'd probably go for the cheapest one, probably mice are cheaper at this stage. If you order some frozen thawed through the mail I'd get a few hoppers, adult mice, large adult, and XL adult mice and maybe a few rats that are a little bit bigger than XL mice. If you have smaller rodents that are left over I'd feed several in one feeding about 10 or 20 minutes apart to use them up. Most of my snakes will eat 3 rodents within an hour or so if I give it to them (depends how big they are).
If I were you I'd feed one or two rodents twice a week for at least the first six months or so until he gets some weight on him. Some times snakes will go on a fast and won't eat for weeks or months, you want to get enough weight on them as fast as possible when they are young just in case they go on a fast so they survive it. I have a snake that went on a fast and she was skinny to start, scares the heck out of me how skinny she is but she is finally starting to eat again (thank goodness!).
I have another large 1400 gram snake that has eaten very little over the last few months but you can't tell he hasn't eaten because he started with a really good body weight. Ball pythons can be very finicky and when they eat it's best to feed them as much as you can while they are in the mood. I've heard of ball pythons going on a fast for up to eight months at a time! That's pretty much normal.
I bought my rodents from Rodent Pro dot com, but any online place will work, just shop around for the deals.
Snakes will always show signs of hunger and will eat almost all you give them. Most would eat every single day if you offered it to them, but that's way too much LOL.