It's not cute and it's not funny.
Sexism is normalised by stupid, offensive T-shirts like that. The only men that sort of sentiment appeals to are losers.
If your son wore that slogan to my DC's no-uniform high school he would be told it was inappropriate. Because that is what it is. He would spend the rest of the day wearing a standard issue PE t-shirt. Repeat performances in the offensive shirt would earn him an intervention by a dean.
Wearing this kind of moronic shirt is not 'embracing his masculinity'. The slogan is perpetuating a very narrow and very limiting version of masculinity.
It's the 'masculinity' of football hooligans, men who think it's ok to piss on the street, wifebeaters, and rapists.
Carry on if this is what you want for your son. You are doing him and everyone else in society, boys, girls, and decent men and women alike a huge disservice.
chess is about 85% male and many chess players (more than average bunch of people) are sexist and not it a jokey way, I think they actually believe it.
Of course they actually believe it.
Why do you go along with this by buying the shirt that parades this shit as acceptable?
You need to start joining the dots here.
Homophobia and the sort of masculinity signalling you have aided, abetted, and encouraged in your son (i.e. open misogyny) are related. The only masculinity that is acceptable in this culture you have encouraged is the masculine culture that says women (and what they see as 'feminine' men - gays for example) are inferior and 'real' men are entitled to treat them badly, and men who respect and treat women as equals, and get their own fucking sandwich, are letting the side down.
It is all done as part of an exercise in masculinity signalling by males who are deeply insecure, do not feel confident with girls or women, and very often turn into angry men who end up causing untold misery to women.
Or they end up undateable and turn into bitter, angry incels.
It's not a 'quality' look. It looks absolutely pathetic.