You should charge him! I don't know how much because I'm not a game artist but I do lots of photography for companies, also small ones and I always ask them for some kind of payment in return. Like a favour, money, advertising, something I can use. Never do it for 100% free, especially if the customer wants to sell (a part of) your art later on.
There also are different kinds of commissions. First decide whether you want to allow him unlimited or limited use of your work, commercial or non-commercial, etc. etc.