1. Combine first 3 ingredients in a small saucepan; bring to a boil over medium-high heat. Cook until reduced to 2/3 cup (about 10 minutes). Remove from heat; cool completely.
2. Combine cooled marinade and pork in a medium bowl. Cover and marinate in refrigerator 2 hours.
3. Prepare grill to medium-high heat.
4. Remove pork from bowl with a slotted spoon, reserving marinade. Place marinade in a small saucepan; bring to a boil. Reduce heat, and simmer 5 minutes. Cool slightly.
5. Thread 2 pork cubes, 2 red onion pieces, and 2 pineapple cubes alternately onto each of 12 (8-inch) skewers. Lightly coat kebabs with cooking spray. Place kebabs on a grill rack coated with cooking spray; grill 3 minutes on each side or until done, basting frequently with marinade.