Most Helpful Guy
Casual usually means there is no commitment to each other, and each partner is usually free to see other people. Not a boyfriend girlfriend relationship. If you guys are more boyfriend girlfriend monogamous, then it's more serious.
Most Helpful Girl
If he says he's starting to develop feelings then he probably does want something more than a casual relationship. The best thing to do is talk to him about it so you can figure it out, although it does sound like to me that he wants something more than just a casual relationship.