The time from being exposed to the virus and becoming sick can range from 12 to 25 days but is most commonly 16 to 18 days.  I had it as a child and had it for 3 weeks and then it seemed to get better for a few days and then i got it back worse and continued for another 3 weeks. The doctor said the first time i didnt get full dose and thats why i seemed to get better and the i was reinfected and got a full dose second time round.

If i was you though i would take him to Docs just for peace of mind.

Hugs Tee