Is it wrong of me to think of backing out of meeting an online friend after years of talking about meeting up?
Why are you thinking of not meeting now?
I don't know, I'm just afraid of meeting him
It is understandable to to be nervous after all this time.  But, it would be very rude to cancel the plans.
It's just that I don't really want to meet up with him.
If you have made plans to meet up with someone, you should follow through with it. It's wrong to cancel on the person and this will likely hurt the relationship.