I wouldn't jump to conclusions based on someone's text asking him if he's gay. If you didn't hear it straight from the horse's mouth; then you didn't hear it at all.

What you need is an upfront conversation. You need to tell him you snooped on his phone; saw a text; and it's been bothering you ever since.

If he says he is gay; well; then that's something the two of you can work on moving forward.