http://www.nhs.uk/Conditions/vaccinations/Pages/measles-outb...

"Can you still get measles after the MMR vaccination?

It’s extremely unlikely, but you need two doses of MMR to be fully protected. The first dose of the MMR jab protects 90% of those who receive it, and the second dose tops this up to 99% protection. Almost all of the children in the Welsh outbreak who caught measles were either completely unvaccinated or only partially vaccinated."

Assuming everyone was vaccinated then:

2000 people exposed, with 99% effectiveness of vaccine, yields about 20 people who will contract measles despite being vaccinated.

Many vaccines build "herd immunity" and will not work as well if people decide to opt out for some frivolous reason.

Also of course some people cannot receive a specific vaccine because they are pregnant or immunocompromised. Others should be vaccinated to prevent spread to those vulnerable populations, not just to protect themselves.

But seriously, children don't usually get vaccinated for measles until they are 1yo.

Because young children can not be vaccinated, so he should stay away from them for a time (don't know how long).