Here is a way to test their sincere interest.
Tell them that your son has a very busy schedule and can't stay for the whole camp. He has the time to come in and pitch a few innings and leave. If he can fit this into his busy schedule would they pro-rate the cost for his time spent there? $50 or $100?
Per NCAA guidelines they can not make the camp free, but at least when my son was being recruited a college did just this. Their camp was a two day event and my son came in, warmed up, pitched and then left. I don't remember the details, but I think he paid 25% of the fee.
Like others have speculated they have seen enough of him to have a pretty good idea what they want to do so skipping the camp probably has little impact.