If I were the mother, I'd say to him

 

"Son, thank you for the fish.  I'll enjoy it even more knowing that you were thinking of me while having a good time there.

Next time you are in this situation, think more about your responsibility as a guest.  Your presence should enrich the experience of your friend and the host family, just as their invitation enriched yours.  Show that you are having a good time, and your happiness will brighten their day.

We may be poorer material-wise, but in spirit, we are equal.  They are very generous, not because they are rich, but because they have a generous spirit.

When you are there, you represent yourself.  Focus on what you need to do.  I know I am always in your heart, even without the fish."