You'll need to use some sort of Remote Desktop licensing if you want to remote on while he's using a current session. Standard RDP is limited to one physical or remote user per PC. Most remote sessions will either interact with the same session (LogMeIn, Teamviewer, Google Remote, RDP, VNC, etc) or you'll need something else that can allow multiple sessions between physical console access and remote console access. If you remote on using RDP while he's using his PC, he'll be booted to the login screen and his account locked (which will leave all his stuff open). If he logs in, your remote session will be closed.
The rest of those pieces of software will be you remoting on and interacting with him and his session. Again not what you want.
Windows Server can do remote sessions, but requires licensing to do so, and is far from what you want to do I imagine.
NComputing got round this with some software...which I have a copy of the disc somewhere because we still deploy a lot of their thin clients when offices want to cheap out on workstations. Not sure if they offer their software for purchase without a thin client...but it does work without the thin client. And really is the quickest way I can think of to have two sessions working independently on the same PC.
Edit:
You can make a free account at NComputing,
https://www.ncomputing.com/en/support/software
Edit Part Deux:
I didn't think to ask before...but if you're planning on gaming through this connection, odds are you'll be sorely disappointed. For general browsing and work use, it should be just fine though. You might even get away with decent multimedia streaming if your network can handle the load, but RDP isn't really meant for heavy media streaming...but I have streamed over RDP before and it does work. So depending on your usage plans, this might not be the route you want to take...