Hi Gianna,
Thank him for the like he sent you on the dating site. Crack a joke about something fun the both of you shared in the past while hanging out and then let him know that you would rather the both of you stayed friends as you don’t feel a romantic connection.
That is really it.
It makes your message lighthearted and fun, but it also lets him know that there is no potential for romance. Be polite and honest without ambiguity.
Guys really appreciate it when a girl is upfront about how she feels about them instead of being ambiguous. They respond to honesty a lot better than ambiguity.