Mleon it could be a money maker for the program. It could be serious interest and they really want to see your son and get to know him. There are two ways to find out.
#1 You could call and ask them. "Hey coach money is tight right now. My son is interested but we don't want to attend unless there is serious interest in him." Of course they could come back and say "Well thats why we invited him. We want to see him and evaluate him and see if the interest is warranted." Or it could turn them off and they could be thinking "Heck we give the kid an opportunity to come and show his stuff and he wants to know if there is interest."
#2 - Simply go and let your son show his stuff. If they like him they will let you know. If they don't you won't hear back from them.
In this whole recruiting, finding the right fit, blah blah blah thing there are risks. Sometimes you just have to do it. You have to put yourself out there and see what happens. Your not going to know until you go and find out. If this is a school your son is truly interested in attending and a program your son thinks he would like to play at then go and find out. If not pass on it and find another opportunity.