6 months used to be the norm and so vets were trained that way. Now 4 months is becoming standard with pre -pubescent neuter now becoming common in shelter environments. we neuter at 3 months (i kilo) if healthy at Yorkshire Cat rescue but that's partly so we can be assured kittens are done. They recover easily at this age and our vets are very used to it (we send a lot there way!!)
don't worry to much about unwanted behaviours though as boy cats mature a little later than girls and so he shouldn't be testosteroning until after 6 months. if we have boy cats with un-descended testicles we wait until 6 months
good luck with the chop Mully!