Ukrain was part of ussr back then so why should he fight on the nazis side

It's called collaborationism. Countries that are invaded often have people on the inside of that country who side with the invaders, for opportunist, ideological, and/or material reasons.

Nazi collaborator monuments in Ukraine - Many new streets and monuments have been erected since a new government took over in 2014

Part 1: A brief history of the OUN-B nazi-collaborationist faction from which current day neo nazis are descended (CW: images of pogroms, sexual assault, dead children, and the holocaust)

Part 2: A brief history of the OUN-B nazi-collaborationist faction from which current day neo nazis are descended (CW: images of pogroms, sexual assault, dead children, and the holocaust)

PDF of declassified CIA operation aerodynamic which leveraged ukrainian nazi collaborators against the soviet union after WW2

Even if he managed to flee to Germany they would have never even let him get such a rank, because he wasn't German.

https://en.wikipedia.org/wiki/14th_Waffen_Grenadier_Division_of_the_SS_(1st_Galician)

The 14th Waffen Grenadier Division of the SS (1st Galician) was a World War II Nazi Germany military formation made up predominantly of military volunteers with a Ukrainian ethnic background from the area of Galicia, later also with some Slovaks. Formed in 1943, it was largely destroyed in the Lvov–Sandomierz offensive, reformed, and saw action in Slovakia, Yugoslavia, and Austria before being renamed the first division of the Ukrainian National Army and surrendering to the Western Allies by 10 May 1945.