Question about screen sharing

I am planning on helping my grandfather use his first iMac. He's used windows xp, so clearly OS X may be a challenge for him initially. I know that I can take control of his screen using iMessages on my iMac to assist him if he's having trouble. However, if I'm not next to my iMac when he needs help, is there a way I could use my iPad to help remotely? Is there an app that would allow me to use my iPad to take control of his iMac screen? Any help would be appreciated.

As a side note, any tips on setting up the computer for an elderly person? Besides the obvious such as making the font bigger are their any other ideas or tips? Thanks.