I would guess he'd also be learning German there too? Can you speak with him at home?
If that's the case then go for it. I think it would be awesome and would love to do it for Heathen.
We're working on teaching her German now. Just introducing 1 or 2 words every month or so.
No personal experience but my friends DD goes to an immersion school and is doing great. She did 6 months there before it became full time immersion.
I don't think scores matter as much as your personal impression of the place, visit it and ask questions and see if it feels right. Look at all aspects of the school. Research is pretty clear that being bi and multi lingual is positive so if you like everything else about it then i'd go for it.