How can I tell what his motives are?

So, I've been talking to this guy for a few weeks now. We talk online pretty much everyday, and we text sometimes too. I don't see him too much at school, but I talk to him when I do. He's always really sweet. He tells me I'm beautiful, he can't wait to see me, he misses hanging out with me, etc. He told my friend he liked me and wanted to take me out, and he asked me to prom and everything. Now this chick from my school who I don't really know is telling my friend that I should be careful, because he treats lots of girls this way, so he can get in their pants. How seriously should I take her warning? I am in no way planning on letting him or anyone else get in my pants, and if that's why he's sticking around, I'd like to figure it out so I can move on before I get hurt. Now that I know I should maybe look out, how can I tell if that's really what he's after?