If you are a little interested in him and he is your second choice, it wouldn’t hurt to actually go out on a date with him.
Dating apps give you the opportunity to meet a number of potential matches. Who is to say that you won’t change your mind about him once you meet him on a date?
If you aren’t convinced and still don’t want to go through with the date, just let him know that you don’t believe that there is a romantic connection and wish him the best in finding a compatible match.
You don’t have to explain yourself, even if he asks. Just be polite and to the point. Guys appreciate straightforwardness and though you feel that you are being cruel by doing this, it is a lot more cruel to lead him on.