The letter is from a lawyer, not a court. It can be ignored. However I suggest sending a registered letter back to the lawyer to waste their time.
They will not spend the $20,000+ needed to go to trial. (That’s only the court costs that must be paid. Full lawyer fees will be higher) I know this because I once had to sue a contractor. Court fees would have been larger than any money I would have gotten back. Fortunately it was handled through state licensing.
The letter should reference that your project is using the English word that describes the function.
I went through this decades ago because my Internet company name closely matched an extremely large computer manufacturer. I got a letter from an attorney. I wrote a letter back that my company name was the English word for the equipment used for Internet service. That was the end of it.
You are wrong that he should immediately change his app name.
This is a letter from an attorney. It means nothing. It’s legally the same as if I got my buddy to walk up to you and say, “Hey, my friend thinks you have his car. Give us your keys.”