'if my son has a VPN installed on his computer....'
i would get him to remove it. not just because he could access adult material but because free vpn's (paid need a debit/credit card) are notorious for scams/hacking purposes. there are 'good' ones typically web browsers such as opera has a free one. most free ones though are harvesting personal info to varying degrees.
If my son has a VPN installed on his computer, will the child safe on websafe security block adult websites?
Just because he has a VPN installed does not necessarily mean it is being used to access inappropriate content, for example it maybe used to access appropriate content that is geographically locked to specific location. If it is being inappropriately used then its removal alone will not solve the problem and other stringent controls would be require, for example limiting use of devices to commonly d frequent spaces at home.