jkklpu - what gives you the impression I don't speak Spanish? I did study it to degree level many years ago and I did teach it at KS3 - you don't have to be a native speaker to teach a language to a certain level!
DS enjoys learning a bit of Spanish, but we don't do it in a formal way. Very often he starts it and I then involve him into a conversation that is fairly repetative in a joking/fun way.
He knows the numbers to 100 and feels very proud being able to do maths in Spanish, but I reconsidered practicing X tables in Spanish as he recites/remembers them in German - it would just make it harder to make them stick.
We will still do +/- in Spanish once in a while though.
If you don't really speak Spanish, it's quite a lot to expect him to pick it up accurately when he's already in a bilingual household. If you insist on him doing it, why don't you set up a conversation class with a native speaker so you know he's exposed to correct Spanish? Seems a bit odd to be pushing it in this way, tbh.
We are a German/English household, however I have started doing a little Spanish with DS (7). He is taking it on really well and we have practiced maths in Spanish (+ and - up to 100) a couple of times.
Now he needs to learn his time tables and do division and I am not sure how to do that with him.
Do you (always) say, e.g. 5 multiplicado por 2 or 12 dividido por 2? Or are there other/shorter ways of expressing it?