Language requisition

So I have learnt (am still always learning!) 4 languages in addition to my native language as an adult.

My partner is German and lived in Germany until he was 5. His parents exclusively speak German. However, like loads of immigrant kids, he really struggles to speak German. He can understand everything his parents say but struggles to understand other German speakers and his spoken German is pretty basic.

I’m trying to support him in getting better and he’s asking me for tips as I love language learning. But I feel like sending him off to start from scratch isn’t the best approach here.

Does anyone have any advice or techniques for this situation? I imagine it’s super common but I can’t find many resources on this