French, aide moi, s'il vous plait?

Sa and son are "his", depending on the gender of the following noun. Since the nouns are different genders, you would have to use it twice (sa tante et son oncle). You wouldn't use leur because Guillaume is singular. You would be saying "Guillaume is looking for their aunt and uncle", which, in English, doesn't make sense. So, your solution is to say "Guillaume cherche sa tante et son oncle." =D Hope this helped you! =)