Answer to "enjoy your meal"

Since there is no set phrase/cultural equivalent in English for Bon Appetit (you can say "Enjoy your meal", and it would be understood, but it is not what people say' when starting a meal and is pretty uncommon), there is even less likely an accepted reply.

If I were to describe a classic British meal, the appropriate response to something so unexpected would be stony silence and reflection upon ones past sins as though one were climbing the stairs to the gallows. Any more substantive response would get nervous glances and an inner reminder to avoid playing darts with them.

But one could have a smoother, close to literal translation that could work (e.g. something like "Please join us"). It just wouldn't be idiomatic.

No, I don't think English has a comparable common phrase.

However you could always just ask him if he's interested in joining you.

Would you like to join me?

or

Care to join me?

or

Care to sit down?