Do not make a 19 yr old boy dress like a middle aged tourist please.. honestly.. he will totally stand out then. He doesnt need to look like Arnold Palmer.
most younger guys were tight fitted jeans or chinos.. and their Tshirts are more fitted than most north American youth wear.. but there are always exceptions.
Let him be.. museums are hot and crowded with masses of tourists.. and they are wearing stuff from allover the world.
Stop worrying about the locals.. they arent worried about you.
Basket ball shorts arent "in" but who cares.. if you let him take a pair he might decide to change out of them himself if he feels uncomfortable.. just make sure he has a pair of regular shorts with him..
When I took my 13 yr old he wore those baggy jeans( this was a few years ago) .. I PICK MY BATTLES , and I wanted him to enjoy the trip not obsess over his image. We were in Paris for about 3-4 days when he asked if we could go shopping for some narrow jeans.. lol.. his choice, his spending money. Okey dokey no battles over packing and whining about wearing stuff they arent comfortable in .. let him pack his own stuff.
He should have one pair of decent jeans to wear out to dinner at night if going somewhere decent.. ( yes jeans are fine as long not ripped torn crappy looking ones) and if its hot some lightweight chinos if he wants.. and one decent looking shirt.. short sleeve shirt or a tshirt with some style ( like a nice v neck one thats fitted).. other than that let him be.
I have seen some freaky styles wore by teens in Paris.. so dont stress about it.
Comfy shoes or runners a must..