I can see that I need a single user license for his laptop but what additionally do I need to buy for the wingate end?
Yes, you would need to buy a license for the WinGate server end as well. Additionally you will need to have an ip address or domain name that your son can always connect to; i.e. a static ip address from your ISP or a subscription to a dynamic dns service.
Also, since I dont have the VPN part installed can I just overwrite the current installation and add the VPN patrs?
The VPN features are controlled by licensing, so once you add the singe user VPN license into your WinGate servers installation, then that feature will become visible.