Not sure what the official answer would be but as far as I know:

• A passport doesn't say where you live.
• Train staff never ask to see proof of residency. They do sometimes ask for your passport/ID card, just to check if name and number match with name and number on the Interrail pass.

Your son would travel for free anyway, so I'd just go ahead and order the same type of pass for the both of you.

Thank you very much for your reply. This makes sense and reassures me much!

Hi Philipp, 
We need to understand if you have any sort of non-EU residence permit/passport, or whether the child has any sort of EU residence permit/passport. If the child has only 1 residence outside of EU and the parents has only 1 residence within EU, I am afraid the only solutions would be to either purchase point-to-point tickets, or a Eurail Youth Pass for your child. If your situation is different or if you would like more advice and assistance, please reach out to our Customer Support team directly via customerservice@eurail.com or any of the other contact options listed here: https://www.interrail.eu/en/support