I don't recall anyone telling him he can't learn to fly a glow plane.
It's possible for some, difficult, or perhaps impossible for many others. Those of us who recommend an instructor do so because we've seen many people spend lots of time and money trying on their own, and eventually fail. We also do so because we recognize the many safety issues involved in learning how to PROPERLY handle a glow plane. What happens when the newbie, who goes to some remote field to try to learn by himself, severs an artery in his wrist because he didn't know needle valve adjustments should be done while behind the plane? Many who have learned on their own later admit it would have been easier and less expensive to get instruction.

So, one can be macho and go it alone, or they can listen to the voice(s) of experience and get help. It should be pretty obvious which way I recommend.