This could be plenty of things, for example an allergy to underwears. You do not know yet if it is an infection, until the results come back.
He should not be tested only for Chlamydia. Actually, there are three tests: one for Gonorrhea, one for Chlamydia, and one, the "NGU" test, for any other type of infection in the urethra.
If an infection is confirmed, you should both be treated. You can be the carrier of the germs and still test negative.
Hope this helps,
Francis.