Pluto meets in all four Parks and at several Character Meals too throughout WDW.
Character Meals: Garden Grill (dinner only), Ohana (breakfast only) Chef Mickey, Garden Grove (Swan and Dolphin)
Animal Kingdom - He meets with Goofy in Dinoland. 11am - 30 minutes before Park Close. He's also been at the Dinoland Dance Party. Check times for the dance party. It's near the Goofy and Pluto Meet Area.
Hollywood Studios - He meets with Goofy in front of the Sorcerer Hat. They start meeting at Park Open and usually meet until 3:30pm. They do swap out with other characters through the day but only for 20 minute sets.
Epcot - Legacy Plaza as you enter the Park. Looking at Spaceship Early, he's to your right. 9am - 12:45pm and 1:40pm - 4:15pm
Magic Kingdom - By the Flagpole as you enter the MK. 9am - Midday and 1:10pm - 2:30pm. Pluto meets during morning Extra Magic too.
Check out Kenny the Pirate for up to date character hunting information