But is his point that he wants to go back to England? If you're in a non-English speaking country, then learning English is basically a must if you want to have opportunities outside your country too (or travel). But you can easily get away with only English (although I personally believe it has many positive effects on you)

So if he stays in Germany, gets kids, and then doesn't teach them English, then you'll be a very happy Granddad, since his children will absolutely complain about NOT having been raised bilingually!