From what you describe so far, I wouldn't be so sure if this is a connection. The surname Pogulyaev is much more common among Slavs, as are the names Vasil and Danil. If there is no connection to Novozybkov / no Jewish ancestors, then it's probably just a false lead.
You can safely tell your father that his father went MIA in September 1941 and never found, but clearly killed by the Germans, and his mother was living in Kiev in Podol after the war was over. That his likely uncle Zinovy also perished in the war. That there are historic documents left in Belarussia's and Ukraine's archives which will probably connect you with the "DNA relatives" in a clear way, but that it is a search beyond the capabilities of a few internet-savvy helpers (an actual archive dig will be required!)
How did he end up with his mother's surname in 1947 is an interesting topic to broach. Did he know the name, and in what context.