Look, I understand you want to close the gap, however, there are TONS of other options. You're grabbing after the easiest straw with the most severe longterm consequences, which is clearly not healthy.

We have an apartment contract signed

Why didn't you go for separate places first? Your bf would benefit a lot from living in a flatshare with other people first, that way you can date "normally" and see if you're actually a good match in person before moving in together.

He has started to learn the language

So how will he make it to a comfortable B2/C1 level? You realise it takes more than a few months, right?

Everything should be settled..

What if he hates living in your country? What if he feels lonely?