"Hey man, sorry about the texts. It wasn't a good forum for this conversation and I think I may have come across as rude which was not my intention. I'd like to talk about the towel/mat situation and come up with something that everyone can be happy about. Do you have time now, or is there another time that would be better for you? I think [third roommate] might like to get in on the conversation, too."

Or something to that effect. Maybe he won't be amenable to finding a solution, but that harkens back to remembering that you can't always get what you want. Do you want to spend your time being annoyed about it and disparaging your roommate? Or use this as a lesson the next time someone wants you to acquiesce to a compromise that you don't like?

Just be wary of deciding that you're "taking the moral high ground." You might be right. He might also be right. If you let yourself be smug about being 'right', especially when it's some shit like where you hang your towel, you might very well just turn into a dick.