My 4 yr old likes anything pink, but isn't really "obsessed" -- she's fine with any other color too.
This is kind of random, and very forward of me, but an "obsession" that lasts for so long in a kid so young doesn't seem "normal" (given that normal is a pretty broad range). The only child I know with that degree of "obsession" over something was recently diagnosed with autism. If he has several things in his life that need to be routine or the same, you might look into some beginning screening tests, or at least questionnaires. Your pediatrician could point you to some resources, or even the school district (the school district will work with kids from age 3 and up). There are also several resources online: