He does bring up a legitimate concern that should be voiced, and since you are the one handling this hiring, and claim to be the developer, it is only natural that we voice our concerns to you directly, or on a forum where others can hear the response. With that being said, let me explain something.
One thing you fail to grasp here is that we, as developers looking for work, want to be secure in that employment. We don’t want to have to worry that the group will go under, or you’ll get bored and move on, or any number of scenarios where we don’t get paid. Based on your profile and current activities, plus the fact you mention we are doing this as volunteer work makes this about as unsafe an offer as can be. Developers want to be paid, not given a pat on the back. If you cannot offer any guarantee of payment, with your current situation as outlined in @Pavalineox response, then you should expect these kinds of comments.
Please either fix these concerns or accept the complaints as legitimate, but do not tell people to keep quiet over them.