Beavertonite is correct, I strongly admonished him to stay on his back, in fact it was the first thing I told him to do after I ran over to him. Unfortunately, being the only licensed ham operator on the scene, I was busy on the radio when his son and another individual helped him onto his feet. I yelled over from the radio, but that's about all I could do. At that point I allowed him to sit in the front seat of the Xterra. I even offered blankets and such to cover the ground to lay on, but it's hard to be in two places at once (though I do have an identical twin living in NJ).
Given the choice of providing immediate medical care versus emergency comms, I knew I was the only one who could make the call, and I tried my best to give directions from there.
Recently there has been research refuting the original thought of spinal immobilization and backboards. Yes, if there is spinal injury moving about can lead to more damage, even paralysis, BUT spinal immobilization via C-collar or by other mechanical means is showing to be less effective than once thought. In my area C-collars are going by the way side and are usually only applied in certain conditions meet criteria. If I recall correctly application is utilized when an individual shows neuro deficit after an accident (fall, crash, etc), is ejected from vehicle, motorcycle crash >20mph, auto vs pedestrian >20mph, axial load to the head (such as diving head first into shallow water), or a fall greater than 3x the person's height. In addition, backboards/spinal boards are showing to be liable to causing neuro damage as well since providers and EMS workers are likely to cause injury simply by the mechanical means of application, and the time spent on the board obtaining CT's, x-rays, treatments while waiting for those imaging results.
As for holding a patient down, you are on the verge of crossing several lines here as well. Naturally your first instinct is to want to cause no harm, and as long as you are working in those boundaries you should be free from tort. BUT, if you hold a patient against their will, you are at risk for a civil suit (false imprisonment/held captive, etc). If a person is eliciting signs of confusion or is unable to make reasonable decisions for himself you are likely, within the law, safe to hold them against their will. Now, if the person has sound mind and judgement and wishes to go about his way there is nothing you can do and therefore you should not be held liable for any injuries he causes to himself. I can't say I've ever encountered an individual being sued for holding a patient against his will in the hopes of stopping further injury, but I have seen plenty of injured individuals refuse care. They always mutter the same similar phrase, "...if I need it later I'll have someone take me in"....lol.
Be forewarned, I'm no lawyer or legal expert, but I have been an ICU nurse for 12 years and a flight nurse for 5. Sounds to me like you did everything right. You assisted the injured, attempted to reduce further injury, and called for help. Aside from a CT machine and some bandages I'm not sure there is more you could have done. Good work and good job!