Remote Desktop

I would like to help a relative who is experiencing some computer problems who lives in a different state.

About 6 months ago I visited him and setup a wired/wireless network via a cable modem/router. His computers are behind the router and firewall and are using internal IP numbers.

Is there anyway that I can “remote desktop” into his machine to provide some assistance instead of driving the 4 hours to get on-site?

Any advise would be greatly appreciated.

Thank you!