Your son does not need a suit. What you are suggesting is just fine. Dressy capris are usually allowed but shorts are shorts and they are not allowed in the main dining room during dinner.
In saying that, there seem to be more and more people dressing down these days and I think that's a shame. However, depending on ship etc. they do say no shorts.
Dec. 13 - Windstar sailing yacht. Wonderful!!
Just completed - Celebrity Reflection, Nov. 22 (learning to sail solo) followed by Independence of the Seas Nov. 30.
44 cruises - too many to list however cruiselines are in no particular order:
and some unknown tub
Yes, I'm a Royal Chump and proud of it.