This is going to sound really pessimistic and scary, but I don't mean it in a bad way. I just wanted to say, please keep an eye out for any symptoms that could be a clue to his health, and ultimately a clue to his past.
One of the symptoms of effusive FIP is a bloated belly. This can be very serious, so look out for other symptoms - watery eyes, ulcers or conjunctivitis, jaundice, anemia etc.. (Have a look on google - there is loads of info about FIP)
It does sound a little strange that this owner was unable to show you anything about him, and that does worry me.
How much did you pay, if you don't mind me asking?
I sincerely hope everything is ok and he turns out to be a dream pet.
I pick up my baby siamese boy this friday!