We went last year & my son's favorite is R2-D2 as well. R2 was in the Darth Maul & like the previous poster said, he appeared with Luke. It took us several tries to line it up right to get at the front when R2 was there. Luke/R2 would swap out with Princess Leia every 20-30 minutes or so. The line snakes through a corner of the building so you'll likely be in line for a while. We paid close attention to how many people made it to 1 particular character & then we could judge the rotation based on that with our place in line. We had to keep going to the back of the line or swap places with a few people to get the timings right. It is worth it though. It is looks just like a real R2. He's even somewhat interactive. It is a very cool experience. Plan on spending at least an hour in line, but the good thing is it is inside so at least you aren't in the sun.
If your son is really into R2, then you might also want to go see R2-MK. This is the Disney version of R2 that appears with the Disney characters that are dressed up like SW characters. Again, very interactive & looks authentic. This guy is a riot, he 'drives' himself through the park to get to & from the photo location. It is a sight to see. R2-MK appears outside & towards the center of the park. He appears with Jedi Mickey. Again, there's a rotation here with characters, so timing is important, but worth it. Those were probably 2 of my son's favorite characters to meet.