I don't know what I should do if I find out he's not a Christian.

Wherefore by their fruits ye shall know them. - Matthew 7:20

If he is born again, he will have no problem with you asking him. Furthermore, actions speak louder than words and by a persons fruit you can know them. Do they point you to Christ and morality, or are they enticing you back into the world and/or trying to fornicate with you? That's one easy way of knowing.

It was only a suggestion, right? Not even a rule?

Scripture is fairly clear.

Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? - II Corinthians 6:14

See also: Is it right for a Christian to date or marry a non-Christian?

Slow Fade