First try and get a meeting scheduled with him early (maybe before school starts). If that is not successful, go to a board of education meeting and ask the board if it is against district policy to have a PTO associated with any of their schools. Explain your position and your proposed plan (make sure you have a clear plan of what you want to accomplish your first year and how)
Call the other schools in your district and find out if they have PTO's.
I, too, would want an official meeting with the principal. He may have what he believes are very good reasons for not allowing a parent group. By meeting with him personally, you can cite statistics that show the benefits of an organized and active volunteer group.
He may have had negative experiences with groups or individual parents that make him worry. Speaking with him about his concerns can help you focus on what type of group will help ease his mind.
A well run, organized and active parent group can have a hugely positive impact on the community and actually work to make aspects of the principal's job easier.
A clique of overzealous, gossipy, status hungry adults can be nothing but a nightmare.
His comment, "If he heard anything he'd let you know..." would concern me. Who, exactly, is he waiting to hear from? Sounds like he's stalling.
Overall, perhaps you could start small, slowly building a volunteer base. As he sees the positive impact your work has, he may be more open to an official parent group.
Some people come into our lives and quickly go. Some stay for awhile and leave footprints on our hearts. And we are never, ever the same."
"The ultimate aim of karate lies not in victory or defeat but in the true perfection of one's character."
Worst case you may have to (after trying every other avenue) move up the chain to his boss, likely the Superintendent of Schools. I would (again, after exhausting every other option) set up an appointment with the Superintendent and discuss why there isn't a parent group at the school and why you have been met with so much resistance. Hopefully it won't come to this, but it sounds as though it might. Hopefully you can make the Principal understand the benefits and how this group will be able to support many school activities, such as Field Trips, classroom materials and other items that would directly impact the Principal's priorities.
Another option I would try prior to going above his head is seeing if there is a school council at the school (more than likely there is). Usually a school council must consist of both teachers and parents, and normally in even numbers. I wonder if this Principal is following the rules regarding this. It is also very possible that the school plan is online. Often the school plan will include things, such as the make up of the school council, the verious planned improvements for the year, etc. My children's school plan includes things that are directly affected/provided by the PTO, such as Field Trips and the reading Incentive Program. With what you have described about your Principal I have to wonder whether he is adherring to the school plan and town's School Committee processees.