I’ve seen a few people speculate that he’s a Shimotsuki - what makes you think that?
It's looking pretty likely that Koshiro, Denjiro, Tashigi, Kuina and maybe even Zoro are all Shimotsuki's.
Koshiro's quite obviously the son of Kozaburo who left with the exiles.
Denjiro's the spitting image of Koshiro prior to his perma-madman face. I'm also led to believe that all their names imply this but I don't speak Japanese so can't confirm this
Denjiro's sword prior to going into hiding looks extremely similar to Wado Ichimonji, which is a Kozaburo made blade.
Small one, but when Tashigi (who as we know looks really like Kuina) had her hair slicked back again, she looked like she could be Denjiro's daughter- not saying she is, just the two characters look really alike.
And Zoro's been heavily linked with all these characters now, and Ryuma, Yasu and Toko and to boot still looks rather similar to Ushimatsu's silhouette. Plus he lived in the village founded by Shimotsuki exiles.