Well today, I saw him carrying his little toy bus around and I asked him to say bus figuring he'd ignore me, but instead he smiled and immediately said "buu" "ssss" clear as day. So, I grabbed my iPhone and attempted to get him to say it again hoping to record it for prosperity. There was lots of singing, a bit of signing, some coaxing and a touch of tickling, but I finally got it!!
p.s.: In case you missed it, he says it at the very end of the video when he turns away from me. :)
p.p.s.: When I first start singing Wheels On The Bus, I wanted to point out Leo does say "round and round" just not very clearly. I think we miss a lot of his words because of how hard it is to understand him. I'll be writing a post soon about the apraxia diagnosis he just received that will help explain why the few words he does say often are very unclear so we miss them as well as why speech is so difficult for him.