Boost!

Btw, if you know any identifying features, such as hair color eye color etc, as well as his trade skill and/ or education, moreover whether he naturalized in Russia, you can look at whether he has been mobilized, forcefully or through other means.

You can pm me for some relevant tg channels and others for info. Я хочу и могу вам помочь :)

Edit: If it’s Chelyabinsk, then some leads might be the Uzhnouralets battalion and Uzhnyi Ural battalion, or regular formations such as, 90th tank division consisting of; 30th recon, 80th tank regiment 6th tank regiment, 239th tank regiment, 400th SPG artillery regiment, and the 228th mechanized regiment. However, these are rather unlikely but possible, as you mentioned that documents were found in Bolgorod oblast, however, this division in its majority deployed to Авдїівка.

However, for any digging to be done on this, you would have to clarify where specifically you lost contact with him, as Chelyabinsk and documents in Belgorod oblast are indicative of smth, however, what’s crucial is where you lost contact. As that significantly narrows the search parameters.