How to let this guy off gently..?

You can mention the phrase "friend" a lot towards him so that it sticks in his mind. At the same time, just set boundaries and let him know he stands in the friendship zone, a zone that you dont want to jeopardize. Your reason can be for the fact you are interested in Jack and also you dont want anybody being mislead, nor do you want the awkward feeling of more "xoxoxoxo" and so on. Another way is to just ease up a bit on the attention you give him. The more attention you give him, it may lead him to thinking your interested or that he may have a possible chance.