Sounds like my s2, we're flummoxed & his teacher is flummoxed too...
Is it that he genuinely doesn't understand stuff? Or is hampered by the language, new situation and hasn't found his feet? That can lead to loss of confidence and that's a downward spiral as they sit there fretting about what they can't do/understand and get even less of it.
Or stops trying.
I think s2 does this but we haven't hit on a solution. He always seems happier when we've gone through stuff together but it's 1 step forward, 2 steps back with him.
I allow PC, PS2 and Nintendo acc to school marks, and this week clearing their room. It's the only thing that inspires my 2.
If you worry and fret and get het-up and cross (I did/do) it makes it worse. What worked for me was the more rational, "You haven't a choice, I haven't a choice, let's work on it, you try harder, I help you and we'll get it done and then have more time for things we like..."
1s in our house during particularly hard times have earned a DS game (if reduced!) Or the promise of some treat.
Y5 & Y6 are the Orientierungsstufe so no change to Realschule I think, what he may have meant would be repeating Y5 or 6, the decision to change only comes at the end of Y6. Afaik, that is the same Bundesweit.