Depending on the closeness of your friendship could you tell him "Hay man I jsut wanted to let you know I have a dispoder, aspurgers and it is often hard for me to tell what my boundries are. If I ever bug you just let me know and I'll back off. I like hangin out an wouldn't want to jeprodize that." My son has aspurgers and an obsession with a person is a very real thing. Do you have another hobbie or like you could get into a little more and maybe that could occupy your time more than this individual.

It doesn't sound to me like an obsession. It sounds like a very normal activity. We all have folks we are drawn to and our insecurities lead us to believe we are bothersome.