Split VPN Tunneling - Remote network drive through tunnel but regular traffic through regular traffic

The title is what I'm trying to do in a nutshell. I've been working on a solution for streaming my NAS-hosted media with my friends over the internet. It's easy enough to transfer files with the built-in interface, but streaming to TVs is the difficult part.

Local Setup:

• ASUS RT-AC66U Router running a PPTP VPN server
• Zyxel NSA 320 NAS running a Twonky media server

Remote Setup:

• PC running Windows 10
• Amazon Fire TV Stick running Kodi 15.2

I can't forward the SMB ports on my router so his Fire TV can connect directly because it's blocked at the ISP level (and a bad idea anyways from what I hear), so my plan was to create a VPN tunnel from his PC to my router, so he can map the SMB shares to a network drive and share them to his own Fire TV.

This actually works.

The problem comes up when he wants to do anything else. Neither of us want him using my VPN for ALL his traffic, just the SMB shares. Right now he has to turn on the VPN in order to stream, then back off again in order to use the PC for other browsing. The solution I found was "split tunneling" where only certain traffic is directed through the VPN, but I can't manage to get it to work. I've tried the suggestion of unchecking the "use default gateway on remote network" option in the IPv4 advanced properties of the VPN, but if that's unchecked I can't connect to the drives even with the VPN active.

My test rig for this at home is to create a remote connection via tethering to my phone. VPN on, box checked, it can connect. Any other configuration, it can't.

And before anyone suggests it, I can't use Plex for this.