Dear Holly, Thanks for your question. I cannot tell you that it is OK. The best time to be protected from an infectious disease is BEFORE being exposed to it. While we would all hope that our offspring would always act in a responsible fashion, data shows that they don't always. In addition, kids may not always realize that the same risks of spreading HPV would be there if they engage in oral sex, which has been growing in popularity among teens.
So, all morality issues aside, I would recommend getting the vaccine BEFORE there is any chance of being sexually active. The CDC recommends it be given between the ages of 9 and 18. I would urge earlier in that range than later. It is not a REQUIRED vaccine at this point, and so whether or not to get it is up to you and your son, after conversation with his pediatrician or family medicine provider.