More than one thing (sorry) but these would be the big ones for him....
I spent the better part of 50 years running from religion and hating it.
My younger self would have a stroke finding out I am Catholic and was shown the way by Him.
I used to hate the classical music my father liked to play and made fun of it. How could he not like rock and pop?
My younger self, having recovered from his above stroke would be back in the ICU knowing I now love classical music and 1930's and 1940's big band and swing.
I used to eat loads of junk! Cakes, candy, soda.... He hated most veggies.
My younger self would have been sad knowing I would be fat in my 30's and 40's from it. He would then delight knowing I would become thin in my 50's (till he learned because I stopped ALL foods with sugar, wheat [carbs as a rule], and now LOVE raw veggies and salads.
I mostly grew up in NJ (north Jersey). Not sure about NOW but in the 80's and 90's the most densely populated state by square mile, and I liked it and the crowds.
My younger self would want to know what was wrong with me now that I have lived in Montana (the least populated state by square mile I the lower 48) for 20 years and LOVE not having traffic jams, not seeing more than 3 to 10 cars on my 30 minute commute each morning and how dislike going back to NJ (prior to COVID about every 3 months to visit my mother in a nursing home) and having to be in the crowds of people.
My younger self would be surprised that I love cats (pets) now and could not imagine my life without them.
In the end my younger self would not associate with me now and would avoid me at all costs! But that is okay, because he will change one day!