Will you send him to the Pacific or to Europe?
Do you want him to be injured?
Is there any particular battle/action you want him to take part in?
Is there a particular event you want him to witness?
It's entirely your choice.
My suggestion is to do some research
find some basic cover-all books about the War and decide which bit interests you and then research further.
WW2 is a HUGE topic. A lot happened.
My tip is to find something that interests you, whether it be the 101st Airborne, or the Marines in the Pacific theatre and take it from there. If you can settle on a particular Division/regiment, the plot writes itself to some degree.
When I wrote my first WW2 novel I chose an RAF Squadron and researched their role during the War, and, then when one of the MCs transferred to the USAAF, picked a squadron and did the same thing. The book practically wrote itself.
Hope that helps.
In the end, it's your story.
Good luck.