How to let down a really good guy

I know this guy likes me and my friend told me he is going to ask me out. He already has once and it just didn’t work out between us. I don’t feel that way about him and when we tried dating it was just awkward. We’re friends and he’s a great person. We just aren’t a good romantic match. I don’t know how to say no. And I don’t know how to remain friends with him and date in the future. What if I start seeing someone? It could be awkward to even introduce a new boyfriend to friends knowing that guy might feel upset or something. I don’t think I’m really anyone to get upset over, but if he likes me then idk?? I just feel terrible. I don’t like upsetting people.