Forgetting a language

Yesterday was my grandpa's birthday. Okay, that is not so world important that I would need to tell everyone, but just wait.
Of course did I call him (like every good granddaughter) to say happy birthday. He was so happy to talk to me for a second and after I told him at least 4 times that I'm fine and not starving he spoke a little bit in a very strong German accent and asked me if I could still understand him.
And yes of course I could. How could you forget a language after just a few months?! But my grandfather seemed to be really worried that I would have problems to communicate with him and spoke just "clear" (as accent less as he could) German.

Can you forget your native language that fast?! I would definitely say no and I even think that you could never forget your mother tongue. Wouldn't it be sad if you forget how you spoke in your childhood or that you couldn't communicate anymore with the people (your family) you spend the first couple of years in your lifetime?

I once met a man, he was about 86, but in a pretty good shape, who immigrated in the US when he was about 16. As soon as he arrived here he stopped speaking German and never picked it up again. So I was talking to him in German and he could seriously not understand me. It made me kind of sad, I couldn't believe it.
Well he didn't seem to care, but I swore myself that I will never forget to speak German.

I probably couldn't anyway since I'll go back in a few months :'(.