I like to talk my dad through games but it can be difficult without seeing his screen. Is there a remote desktop-type product that would allow me to "stream" his screen while in DirectX?
I assume since you are talking about Direct X that both you and your father are on PCs.
What you are looking for is a VNC program. However, the problem you will run into is that DirectX runs on the local hardware it is executed on. Therefore it renders only on that particular computer's video card and monitor directly.
Are you both inside one network (wired) connection or in remote locations (or over wireless)? More bandwidth might be required for minimum slowdowns.
UltraVNC uses the GPU for graphics processing, but it might not work particularly well depending on your system specs and the amount of action going on on the other side. Depending on the game you are running, you might need to disable "WinVNC video hook driver" (in your Device Manager under Display Adapters dropdown) on the computer hosting the VNC connection. If things are running too slow try reducing the color settings in UltraVNC.
It's not really possible. You may be able to do something similar with OnLive.