Hello, I have nothing to do with german but I am involved with spanish, greek and english, and I thought I should share it with you.
I am from Venezuela, spanish speaking and now live in Cyprus where I came 14 years ago with my two elder sons (5 and 2 at the time). At that time, none of us with any knowledge of greek. I spoke english but not my children. I put them in nursery school in greek and as they were very young they picked it up quite quickly. After two years I met my actual husband. We met in english and up to now we still communicate in english. My children speak to their stepfather (my husband) in greek and to me in spanish, and by listening to us talking english and tv in english, they also pick it up. We had other two children, and since the beginning I talked to them in spanish, the father in greek, and they went to nursery school in english, which they had already picked up from us. We never spoke english to them but when they went to nursery they could communicate pretty good.
Now they all have the three languages.
When we are all together like having lunch. I speak spanish, my husband greek, between us english, and the children between them and the father greek, and to me spanish.
It sounds a bit crazy but none of the children has got any problem, and they do well in school (in greek).
The spanish is the most difficult one to keep since there is not many spanish speaking people around, even among them they speak greek.
It is hard to keep speaking spanish to them specially when there are friends around, but even though it might not be very polite to speak spanish to them in front of other people, I still do it. Because if I switch to greek or english that will be the easy way out for them, and they will forget the spanish.
A bit of a long and complicated story but I thought I had to share.
The earlier the children pick up a language the easier for them. It would be a shame that he doesn't get the german from you at an early age. My advice is to speak german to him all the time and not mix with english.
Hope this helps. It is hard but it is possible.