His body is no longer producing hormones, but those that are already in his body take some time to work out.
North Shore Animal League America's SpayUSA is a nationwide network and referral service for affordable spay/neuter. They state:
It usually takes about 6 to 8 weeks for the hormones to subside after the neutering.
Bruce Fogle, D.V.M., M.R.C.V.S relates the results of one study:
Ben Hart at the University of California veterinary school at Davis was the first to investigate the value of neutering male cats. He interviewed cat owners twenty-three months after their cats had been castrated. The owners reported that in nine out of ten animals neutering reduced fighting, roaming and spraying. Sometimes the change took longer than in others, and by Hart's definition a 'rapid decline' meant within three weeks of neutering and a 'gradual decline' meant one that took up to four months.
The results showed that about 50% of the neutered cats had a rapid decline in fighting and aggression, and about 10-15% of the neutered cats had a gradual decline in fighting and aggression. Spraying behavior was reduced faster (about 75% rapid decline, 10% gradual decline).