Cinny's Whinny gave you many good comments to think about.
I am a bit confused.
You stated he was on a trial and then you said the opposite.
So ..do you still have him on a trial? Or....have you already purchased him?
Would love to hear a clarification on the above questions.
Another thing you could do is have the owner or previous owner (whichever it is) come and ride him. See what he does.
The first thing I thought of was a saddle issue. After a couple days of riding in an ill fitting saddle...well think about wearing a pair of shoes that do not fit your feet?!!
If you can get someone out to work with you (a trainer) I would have them make an assesment. Only if the vet says he is okay to have work and the saddle fits.
A vet does not necessarily know if a saddle fits right. So I would ask a saddle fitter to come out and do an assesment and fitting. You may find they have a saddle that will work if your keeping the horse.
I have no idea of your riding experience so I can not comment as to if it is something your doing specificly.
What dicipline are you riding? What is it your planning on doing with the horse?
Drug test came to my mind first....If possible get it done! Just in case you need that for a later reason if it were to find something.
Did you see the owner ride this horse? If so what did they do?
You also stated that your mom tried to lead you and it didnt work but then in another place you mentioned the horse being perfect when being lead with or without a rider.
Last piece of adivce......please be open to the idea this is not the horse for you and it is totally fine to find a different horse that better fits your knowledge base and experience....
Sorry for not clarifying, I haven't purchased him yet but I am already claiming him, sorry again. I claim him because he is at my house and pay for everything he needs. I've been riding for 13 years and have had many horses. I don't mean like "I am the best ever" but I have been riding for quite awhile and have rode lots of horses I haven't had this problem. My mom lead him but he always put up a fight and sometimes he does put up a fight when someone is being leading him when he is tacked up with or without a rider and sometimes even does the same untacked but once I get him going he is just great. I am riding him western but I ride English. I had a professional come out today and she said he is just stubborn. His "owner" rode him first, bareback. He would shake his head but he did okay for her but its not me, he does that to my trainer, my mom, me, my cousins, everyone.