What causes low sperm count in a man who has already had children?

Old age can cause lower sperm count.
Physical injury can cause lower sperm count.
Ingesting harmful substances (some drugs and chemicals) can cause lower sperm count.
Interestingly - too much heat in the groin area can lower sperm count too - which is one reason doctors may recommend a man switch from briefs to boxers when having trouble with low sperm count. The briefs tend to keep the scrotum warmer and may reduce sperm production.