You say he's spent time designing and filling in his profile, I think that shows that he hasn't just "forgot" to change his status to "in relationship". He would notice it, and that status is one of the most important aspects of MySpace. Most people on there who randomly add/message people are looking for a relationship so he will get people messaging him based on that if they think he's their type.
I have seen quite a few profiles that lie about their relationship status, it's a lot more common than you think. Some don't even change it after you ask them why they're misleading you, usually after they've spent weeks flirting with you and suddenly slip the words "my boyfriend" in a conversation. I have always assumed people do it because they are wanting a new relationship but they don't want to end their current one until a new one appears. My assumption might not always be true, but either way I wouldn't get involved with such a guy. I always look forward to changing my status to "in relationship" because it lets people know I'm taken and happy.
There is one semi valid reason he may have his status set to single. That's for people to talk to him, as a lot don't look at profiles or message people who are in a relationship. But still, if they message him thinking he's single they'll just stop talking when they find out the truth.
Now as for your situation at hand there are two ways to go about it. One is to message him from a "fake" account pretending to be someone else ans see how he reacts, but this can be dangerous, especially if you start enticing him to flirt with that profile. Or if he finds out you're trying to trick him, I know I wouldn't be happy. The other way to approach it is to tell him you found him on MySpace and query him about the relationship status and possibly the girls. That's what I would probably do.
You may find out how much he likes these girls by what type of comments he leaves them. People tend to put the people they talk to/like most as their top 4 so no need to "investigate" all of his "friends". :P