It’s really simple. Stop being an easy lay. Translation: don’t have sex with him.
Tell him that you aren’t ready for that yet and want to develop a relationship first.
If he sticks around, then he obviously sees more in you than just a place to dump his sperm.
I know that’s kind of radical advice in this day and age of people hopping into bed together after the first date or two, but if you REALLY want to know if a guy is interested in you for more than sex, keep sex out of the equation. Period. Let the relationship develop some depth and commitment before getting between the sheets.