I would agree with much of what has been shared. If the coaches have had contact and interest with your son the camp is a good idea. Unless your son has something that will make him jump out the players for the most part are all pretty good players and not easy to stand out. These are generally money making events and an opportunity for coaches to look at players they have interest in but for whatever reason have not had the chance to see in person or that they want to see again.
I did view it as a chance to see a coaches organizational and interpersonal skills at work. If the program cannot run a one day camp then I would be cautious of their organizational skills.
Some of the best camps were held by DIII programs. There is probably a higher likely hood of getting on a RADAR at a DIII. There recruiting challenges are much different than a DI school.