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No it's not OK. First of all most long distance relationships do not work at all for adults and especially in your age group. Odds are your sitting there being loyal to him while he is off galavanting around dating or hanging out with other girls. At 13 years old, you have a lot of time to figure out what love is. And don't say you know what it is, you don't. Love is complicated. Do your parents know your dating him? If not you should tell them and see if they approve. I totally understand that you have a lot in common, but by the time you are 17 and he is 21, he's going to be bored and going to college parties and whatnot.

OK first you say you feel older when you are around him, then you imply you have never met, so which is it?

He is too old for you and is probably playing around with you. How can he say he loves you and you two don't even know each other.
I think your 13 year old hormones and imagination is running away with you and I can almost assure you a 17 year old boy's mother is not pleased with her son chatting up with a 13 year old. Do your parents know you are chatting with him?