Spander, human AU.
Uncle Rory spends some time with the Harris's, and bring along his son(?) Spike. Xander and Spike have to bunk together in the basement. Spike teaches Xander a thing or two about man on man lovin'.
For some reason, I can't remember any more than that, and I REALLY want to re-read it. I'm pretty sure it was a stand alone, but if there is more to it, I'd love to read that too!
Found! It's Oh Very Young by apreludetoanend
Thanks for all the help.