Eventually he does pit on his own, although it's been awhile and I'm forgetting exactly how it works. I'm just trying to provide a speedy response because I'm guessing you're about to start the El Cappy race, if you're not already doing it.
If I were you, I'd monitor his progress. Don't just start the race and walk away. Observe how he's going and make sure to tell him initially when to come in, as I'm pretty sure Bob will goof this up if you don't do so.
After the first couple pits, try letting him see if he can do it on his own. If he doesn't come in when you would tell him to (let's say you told him to come in on Lap 13) make sure he's not out there until his tires go red!
Funny, this one is. I finally did El Cappy enduro (A-spec) for the first time a couple days ago, even though it's 2011 and GT5 is out! Great track, but 66 laps is way too long. It was exciting for maybe the first 20-25 laps or so. After that, I grew bored because the Lotus Esprit got a lead over the other two competitive cars (TVR 350C and Protomotors Spirra) which means I was just competeing against one car. I get bored whenver there's no question mark in the race, know what I mean? Thankfully the track itself (and the scenery) are interesting.