Hello, I'm Dr. Deb and will do my best to help you today.
My heart also just breaks when I hear stories like this one:( Thank goodness that he's come back to you.
To specifically answer your question about an x-ray, unless bones are broken and there's evidence of healing, then you won't be able to confirm that Kado experienced any sort of trauma. And, even if there was evidence of previously broken bones, the source of the injury would still be in question, unfortunately.
What you're going to have to do is to win Kado's trust back. This may be a long and slow process but I believe it to be doable.
I'd use a positive reward based method of behavior modification. Basically reward him with yummy treats when he doesn't flinch but ignore it when he does. If he's head-shy (which many abused dogs are), then you'll want to avoid touching or petting him too much around his head and neck.
I'd use a harness rather than a collar to prevent inadvertently causing him to react.
I'd constantly stroke and massage his body when he's relaxed and over time, slowly move up towards his neck and face.
If he's food motivated, you can feed him yummy treats while you gently stroke the side of his face or head but take it really slow. Get him used to taking food from your hand (palm out with the food in it) first before you touch him.
Obviously, you'll want to never raise your voice to him or use your hands in a threatening manner when you're around him.
If/when he's around other people, ask them to refrain from touching his head or body. They can feed him (if he's interested), again, with food in the palm of their hand) but otherwise, I wouldn't let them engage with him unless it's clear that he wants to be petted. Take your cue from his reactions/ body language, etc.
It may take quite a bit of time for Kado to realize that he's not going to be hurt but if you take it really slow, more than likely, he'll learn to trust you.
I hope this helps. Deb