W/O/W/
I am going to tell you a story I heard
Someone once went to a Rav saying he HATED Shabbos, The Rov couldnt belive what he heard and then the man repeated I HATE SHABBOS.
The Rov asked the man he hated Shabbos and the man said, when I was young My parents spent all week preparing for Shabbos getting the best Fish, using the best dishes, Getting the best meat.
The Rov thought to himself, that sounds like a very nice Shabbos.
Then the man said, Every Shabbos my father would make me eat that Fish, I hated Fish. My father wouldnt let me eat anything else until I ate the Fish. I almost threw up every time I ate it.
Then the Rov understood, He associated Shabbos with the Fish that he hated. And told me you dont have to eat Fish on Shabbos.
I have been to peoples house where Chulent was FORCED on the kids (The Kids didnt want to be Fleshing) , ive been told “Ain Simcha elah Yayyin, Dag and Bassar”
There are lots of places to go with this, But first Clothes are NOT Important. As long as he isnt naked what is the Big Deal. If he wants to wear Jeans SO WHAT.
Where does it say that wearing Jeans is ASSUR?
It doesnt, of course. People make up Chumras and for some Chumras are good, but for other people Chumras are TERRIBLE and have the opposite effect instead of making people closer to Hashem it pushes them away.
You need to seperate Chumras from Halacha. Just like the Father in the Story made his son eat the Fish , The son didnt learn to like Fish which was the intention, The Son learned to hate Shabbos