S-90 is said to be developed by Sukhoi Design Bureau but there's no information about it on their website. According to the Russian Wikipedia article
the prototype was supposed to be shown in 2000 at the Gidroaviasalon in Gelendzhik, Russia, but for some reason that never happened.
Here are a couple of pictures from the said article:
Sadly, I couldn't find any more information than that.