I would just add him right away. Since the two of you are dating you may as well. I also like looking through their pictures and stuff to see what they looked like before I met them and some smaller information like previous work, where they're from, etc. but that's just me.
Depends I guess. If the topic of Facebook didn't come up for awhile I probably wouldn't seek them out to add them but if it came up or we were both on Facebook for some reason I'd just add them then when they were around. I didn't realise people thought about this.