I think college one day camps can serve a purpose if the school is of interest and you believe that your son generally fits the bill of the type of player the school would be looking for. It can be a great way to interact with the coaching staff and I would ideally combine with an admission tour and see the full campus/area. Your son should get in touch with the coaching staff ahead of time to get the conversation going, express interest, send video, etc. My son (RHP) attended 4 one day camps during his recruitment process, including the school that he ended up committing too. That one was a direct invitation from the coaching staff and required airfare, hotel, etc. and we probably wouldn't have attended if he had not been actively recruited.
In our experience, the camp formats vary widely and you probably need to be ready to "go with the flow". As a point of reference, my son attended a PBR showcase that partially rained out and by the time it was his turn to pitch, most of the scouts had already left. Needless to say, I was not too thrilled... However, he ended up killing it in the bull pen and got some great social media exposure from a post by the PBR state director. I believe the school he signed with saw that post. You never know when opportunities will present themselves and you/your son need to keep a positive attitude and be ready when they do. Good luck!