?? I am baffled that people think it won't matter to the child, its all about whether the parents will miss him too much.
If your son had a close and intimate relationship with your in laws, then I would say OK, it would be a change for him but not too dramatic.
If however he doesn't see them much, as I suspect is the case, it is likely to be a dramatic change for him and certainly traumatic to an extent.
It is very kind of your in laws - but there is no way I would do this with my 2.5 year old. Could they possibly come and stay with you and help out? Or can your wife go too with him, so that she is there with him?
Do read a bit about attachment in children before you decide. en.wikipedia.org/wiki/Attachment_theory and www.facebook.com/video/video.php?v=409644835766678.
Of course the original studies were of children going into institutionalised care and being separated from their parents. Of course your in laws would offer a much warmer environment than that. But unless your child sees a lot of them, you would essentially be sending him to stay with strangers for 2 months.
I think you should listen to your feelings - you said " the idea of being away from little one for this amount of time really worries me". This is for a reason! Your feelings are there not as a trick but to help you.
In terms of learning German, there will be plenty of opportunities for this later. When your son is four or 5 he can go and stay with them with fewer concerns - he can understand the concept of time and space and that you will be returning.
Sorry to rant - but I feel really strongly about this and am often shocked at how little people understand about attachment in children. Often people say "well they won't remember it, it doesn't matter!" Of course it matters. There is much more to ones psychological make up than merely ones conscious cognitive memories.
Good luck 