Is he still on those antibiotics? You may think its coincidental but it may be a reaction to them. He could have been on them 100 times before and only just reacted to them. Worth a shot?
Hmmm...a few years ago my gelding developed acute stiffness a few days after receiving a pen/strep injection. It came on suddenly (he was fine at lunchtime but by early evening he was propping himself up against the stable wall and very reluctant to move, could barely walk). Vet was stumped and started anti-tetanus therapy (as a 'just in case' - the horse was fully vaccinated - but the pen/strep had been given following removal of a rusty iron nail from his frog). Happily, my horse started to improve after a few days and went on to make a full recovery. The vet reckoned that it was pectoral myositis caused by an adverse reaction to the pen/strep. No head shaking or spasms though.
I don't know if anyone has heard this but there has been research in the USA where they have made a link between Atypical Myopathy and Sycamore trees. Here is a report from the BEVA:
my daughters pony also had a virus with some similar symptoms, meningitis type but we treated as that without doing a lumbar puncture. she recovered after about 2 weeks in hospital.
all the best, I hope he recovers quickly