Deciding between multiple clubs is a difficult process. There are many factors in play which will affect your son's club experience. Some are listed below with details you should know before you make a decision.
1. Coach - your son's coach will have a dramatic impact on his performance, improvement, and enjoyment he gets from the season. Your son should know who his coach will be before accepting a place on a team. If you do not know who this coach is, or do not know his qualifications, feel free to ask the coach or club director. They will be happy to share this information with you.
2. Teammates - Who will your son play with this year? Does he know them? Where does he fit in on the team in terms of position and skill level? If it's important for your son to be the best player on the team and get a lot of playing time, understand where he fits in. If he wants to be surrounded by better players (usually synonymous with drastic skill development), are his teammates better than him?
3. Team Role - The last big piece is to understand what your son's role on the team will be. What are the coach's expectations for his playing time and his position? Does this information match up with what you know about his teammates (see #2 above)? If this does not match up with your son's goals, this is a problem. If a coach guarantees your son a certain position or starting spot, do not believe him/her. It is nearly impossible to deliver on promises of this nature consistently over the course of a season.