First, I would take any diagnosis of EPM from this vet with a grain of salt because he is jumping on one neurological disease when there are many diseases/conditions that can cause a horse to move the hind legs funny. The simple fact that a horse has a neurological deficit does not mean that it has EPM---diagnostic testing is NECESSARY to determine the cause of neurological deficits because conditions that affect the brain and/or spinal cord all present with very similar symptoms. Consider: if you have a parasite causing damage and inflammation to the spinal cord at the area of the 3rd vertebra you will have the same symptoms as a traumatic event that damaged the same area of the spinal cord.
And many times lameness is mis-diagnosed at neurological deficit.
So, be sure that he performs a full exam--lameness tests like flexion tests as well as neurological tests. Then if neurological deficits are found don't just accept an "oh yeah, it's EPM" because that can cost you lots of lost time and money when there is plenty of possibility that something else is causing the problem. You should at the very least have blood drawn and sent of to see if the horse has even been exposed to the parasite that causes EPM. Head and neck x-rays would also be high on the list of diagnostics I would want run if this were my own horse.
The fact that symptoms started 7 hours after a long trailer trip is more indicative that something may have occured in the trailer--head/neck trauma on trailer rides is common. Onset of symptoms with EPM isn't going to happen in just a few hours.
For more info on EPM from reliable sources, check out www.meadowherbs.com/EPMcenter.htm