Animals is always a good one.
Probably something he likes. When my son was about to be one, he was into airplanes so that was the theme. Check on Pinterest they have many ideas.
I did a simple theme like pink and white with pink bows!
My daughter don’t like anything but dancing fruit😂her bday is in August so I’m just doing pastel colors. Pinterest is my bff rn. So many ideas on there.