What uniformed CCA is a good one to pick? My son is choosing between NCC Air and NPCC. My concern is that it's just all about marching and more marching. Anyone can share their experiences?

IMO, Uniformed CCA might not be beneficial for your son as he will already be enlisted into the National Service when the time comes. If he is interested in a certain sport or other more value-adding activities, he should certainly go ahead with it. As when he finishes his studies, these value-adding activities will in one way or another be beneficial for his resume/job interviews.