Contamination Fixation...How Do I Help My Son?

Hello, and thanks for reading this post.

Within the past two years, my high-functioning son (now age 13) has been overly fixated on contamination worries.  It has ebbed and flowed over this time, but remains the single most challenging issue for our household. He would listen intently when others used the bathroom to make sure they washed their hands, viewed certain objects as dirty and unable to be cleaned (doorknobs, cellphone, etc.).  At one point, he would insist on "cleaning" the couch before sitting down with a sopping wet, soapy paper towel. There were constant meltdowns and screaming at others for their perceived lack of cleanliness, particularly his older brother whom he believed (for no apparent reason) was filthy and could not be near him.  This has obviously impacted their personal relationship and has created an underlying tension in the family that is unbearable at times.  What's absurd about all of this is that he's the one who lacks most in terms of hygiene (fingers always in mouth, eats food off the floor, etc.), but these things don't seem to bother him.

Lately, this fixation has seemed to resolve and lighten up a bit, but I don't know why.  He now sits on the couch, maintains closer contact to family members, and moves about the house more freely.  However, he views his bedroom as off limits and untouchable, unless certain rules are followed such as washing hands before entering.  He asks his younger brother who shares the room with him to change his clothes before entering if he deems him "dirty".  He uses an excessive amount of liquid hand soap each day, but only before entering and when spending time in his room.  Other soaps "don't feel right" to him, and one or two squirts is not enough - he can go through an entire bottle in one day!

I don't believe this is necessarily and strictly OCD.  He just has very rigid black and white thinking, and a strong conviction that things are dirty and there are rules to follow, especially as it relates to his room and personal space. It could be that his room is his safe space where he feels the most relaxed and at peace, hence the strong feelings?  

I desperately want to help him.  He will not participate in any therapy or self help as he does not believe there is a problem to fix.  What can I do to help him overcome this obsession and the frustration related to it?  Is this typical of others with his diagnosis? Will this dissipate or change over time?

Thank you in advance for any thoughts or advice you may have.