Weak hind legs

My dog sits suddenly and sloppily, legs splay. It's most severe when eating or relieving himself, though it happens randomly walking as well. After an episode he can be his regular whirling dervish self...doesn't seem to be in pain, but the episodes scare me. He's around two years old...I adopted him from a rescue a year ago so not positive on age. It was a rare occasion, but now it's suddenly pervasive. He was in a kennel for a few days and came home exhibiting the issue. We have a vet visit in a couple days, but I'm worried and looking for advice.