Hi,
I see at least two options.
I. You buy a mifi device for him.
Here is an Amazon.com link.
II. When he is here he can buy the same device from amazon.de (
link), or you could now and have it mailed to his address in Berlin.
II a - buy it and have it mailed to me. He can get it then from me. And we both (your son and I) could get him a prepaid sim card for the device. (But you do not know me and I could be a bad guy and keep the device.)
As far as sim cards are concerned I would either go for fonic.de or Blau.de. They are something like € 10 a month for Internet access and prepaid. So, no plan, just ten bucks per month, (if bucks is an exceptable word for Euros.)
As your son is going to stay here for 4 months and certainly knows some German, he should be able to understand both websites mentioned and make a qualified decision what is best for him.
Otherwise, he can call me and I will help him.
Does this make sense to you? Hope this helps.
Knut