Ragwort- presumably if your DS has very short hair it means he is less likely to have hair to hair contact with another child to catch them? IME, it tends to be the children with long hair, who are forever redoing their hair and playing hairdressers and sharing brushes that catch and spread nits. Also anecdotal evidence suggests there are certain shampoos- coal tar and tea tree spring to mind- that the nits don't like or can't get a grip on the hair, so children using those are less likely to catch nits.
As people get older, they keep more of a personal space boundary which is why nits get less common when you get older. When's the last time you sat at work with your head touching someone else's? Primary school kids do it all the time.
Finally, some types of hair, particularly Afro, is notoriously hard to get rid of every last nit from. One parent came to us in desperation, he had tried everything to get the nits out of his daughter's Afro hair. In the end, we got permission for a member of staff to treat and wet comb her hair 2-3x a week until all the nits were gone, and her hair was kept in cornrows in between treatments.
The best way to keep nits at bay is to wet comb your child's hair at least once a week, preferably twice. That means that if they have got a louse off somebody at school, you can catch it before it lays eggs (and the eggs hatch, and breed). If you find several lice in your child's hair, chances are they haven't been caught that day!
PS our school (UK) sends children home if they have live lice and request that the child is treated, and send a note round reminding parents to be vigilant. Problem is, the child comes back either having been treated, or saying they have (and we can't check), but the parents don't do the second treatment, or they have a hard to treat hair type, or the treatment didn't work due to resistance, and then we're back to square one.