Cat asthma? Youtube has a couple videos of cats with asthma...see if you can find one and if that sounds like his hacking.
Did he have a heart murmur? I would closely monitor the hacking/coughing....record times, locations, how long, etc. I could be being paranoid because Mocha coughed and hacked in her last month and the vet says that was a sign of her heart problems. I really hope and pray that is not what your little guy has!
...Always in my heart... Mocha....Nov. 13, 2002 - Dec. 6, 2013
.... Slave to Bengal Cats....Luna, Mystik, Simba, Komet, Meli,
Spark, and Star
.....and a collection of kittens at various times.