Be honest with him
I haven't been in the situation you're describing but I think it's definitely in your best interest to let him know that you only care about him in a platonic way. It will likely hurt him a bit but it's ultimately a lot better for him to know where you stand, rather than think you may feel the same way. Did that make any sense?:-)
You just have to let them go. Let them know. I tried for months to keep a friend one time. It didn't work .
Agree with True8Wolf but in person is better to see & feel the emotion....If it's a true friendship it'll last