Sleep struggle after 3:30am

Me

My 12 week old sleeps ok until about 3:30am... after that it’s rough. He’ll fall asleep in my arms no problem, but wakes when I put him down. If i can get him down, he only stays asleep for a short time. Ahhh!! Why can he do it at 12:30am... but not at 3:30am?? (He also won’t nap when I put him down)....