FTP. Easy enough to secure, highly compatible with whatever kind of system the person might have... They can download and then upload files with ease.
It's also cheap, since there are numerous free FTP servers out there, and dozens of FTP clients. Easy to find help, since FTP has been around almost as long as the Internet, and is well understood by many. It's simple, so if something goes wrong, it shouldn't be difficult to fix, and it's easy to find help as I already said. It's also fairly low on the amount of network overhead, which saves the company bandwidth and your boss downloading time.
It does require that your boss have the programs necessary to edit the files installed on his PC, but that should be an issue unless it's highly specific software with costly per seat licensing.
VPNs and what not have their place, but I like to go with simple solutions whenever possible. Why build something with 10,000 parts when 500 will do? It's always good to remember the old standbys like FTP. Those that are still around are likely still around for a very good reason. From what you've said, FTP would be a quick, cheap, and workable solution. It also gets bonus points for being easily automated if your boss isn't very technically minded about such things. Imagine trying to walk them through debugging a VPN connection over the phone, and tell me there isn't an appeal to keeping things simple.