I like Buffy's idea of simply asking him.

But beyond that (or in connection with that, depending on his response), limiting or ending the discussion does not have to be rude.

How would you feel about saying something along the lines of: "My feelings for you make it quite difficult for me to participate in a platonic friendship with you. For this reason, I feel that I must limit my contact with you. This will explain in advance why I will not generally be responding to social messages from you in the future. As always, I wish you well."