I am just wanting to find out about a guy who would like to have a relationship with me but I feel he is not telling me everything that I need to know about him

Do you think he is married? A lot of personal info is not available online. What "background" info are you talking about?

What is he not telling you ? Background checks will normally just show if he was convicted or went to prison.

If you want to know about other things, find out where he works, where he lives,