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.
More than 300 days at sea so far with many more to come. (Actually more - one of these days I'll count them properly.)
Celebrity Solstice - Feb. 21/15 (moved out a year due to illness)
42 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.