I was lucky enough to attend both, and my advice is that if it is at all possible, do both.
NASS is much more 'recruitment' focused. You get a chance to talk to your region's admissions team, you get a chance to meet sport coaches, you get many, many chances to have any and all admissions questions answered. At NASS, there is a big focus on doing things that are meant to get you excited to complete your application. In addition, there are many guest presentations from speakers that give a lot of information about the Navy. I heard from active duty officers from every warfare community, and I learned more about what there is after the Naval Academy. It is a lot of fun and I had an amazing time.
At CVW it is a much more real representation of life at USNA. You are paired with one midshipmen and you get to meet their friends, company, and professors. There was a brief presentation at the beginning which is the same as the one you would get if you went for a standard admissions tour and a quick speech from the Superintendent. The majority of your time at CVW is unscripted as you are meant to mostly just follow your plebe around to see what their life looks like. I got to meet a lot of other midshipmen who showed me the wide array of people who are at USNA, and all of them were more than happy to talk to me about their experiences.
In summary, I would recommend doing NASS simply because it is an irreplaceable experience. You get to spend a week at the Naval Academy doing fun things and meeting great people. CVW is great, but if your son ends up at the Naval Academy, he will have a chance to experience everything you will see at CVW. However, if he is not sure if the 'academy lifestyle' is for him, it is a good look into what plebe lives are like.