it certainly sounds as if he is now hypothyroid, which can have serious consequences for kidney function
he needs another blood test, but until you can arrange it I would immediately half his Vidalta dose - overdosing is potentially more serious than underdosing
if you check out some of the other threads on here about HyperT, you will see that treating at too high an initial dose is widespread among UK vets - I suspect your vet should not have increased the dose after only 4 weeks when it was clearly improving his condition
get copies of all his blood tests so you can do your own research - there is an excellent Yahoo group for owners of HT cats