patchwork family visa difficulties-any advice? (Moscow)

Hi, me and my family are planning our move to Moscow in the Fall.
My partner (us/hungarian citizen) has a new job and will get his visa through his ne employer.
We are not married yet (it's planned foe end of this year), so I need an interims solution for myself until we are getting married or until I am able to find a job in Moscow.
Here is now the problem: I would bring my son (from a previous relationship) who needs a visa which qualifies him for school in Russia.
According to my partners new company and relocation support this seems to be a big problem. Since my son and my partner are not directly related.

I would be very grateful for any recommendation or advice. Maybe soemone went through the same trouble.

Many thanks, Chris