My 8 year old son confessed to me last night that he likes to wear girls underwear, including that of my fiance and his older sister...particularly the thong variety. I think he just puts them on and takes them back off. I don't believe he actually wears them anywhere. What would be his motivation here?

Considering he's only 8, he's probably just playing around. Try asking him why in a conversational tone why he does it. Make sure you don't sound like you're accusing him of anything wrong or like he's going to be punished, because that's likely to drive him into a shell and you'll get nothing from him.