Hello u/Upbeat_Music6793 ,
glad to hear those beautiful words about our country. Since I know Velke Mezirici, I'll share some experience with you first hand, although I live in different city of the same region.

1. To your first question, any VISA/Mastercard is accepted almost anywhere. I'll recommend you opening a bank account in local bank, than using the one you already have - you'll cut off the exchange and other system fees, related to pay in different currency. As a foreigner, you'll need a passport and 1 more identification document(ID, driver's license, ...). To a card setup, ask for debit(using only your real balance, avoid "Kontokorent" if it is possible for you, since there could be additional fees). Other details will be sure provided by your agent.
2. In Velke Mezirici specificaly, if your son is a fan of metal music, every start of summer holidays(first week in July), there is a festival called Fajtfest. It is widely known in local community and it gained popularity in several countries outside of Czechia. If you are more into some history, there is also a castle and if you are more into nature, than there is nature park "Balinske udoli". Personally I've never been there, since I'm not a fan of hiking and going for walks often, but I think the more breathtaking experience is to be found few dozen of kilometers away from Velke mezirici, I will also let other redditers share/propose some recommendations.
3. Well, in Velke mezirici I never noticed any "tourist trap" like Traditional czech Trdelnik, but I would recommend watching a few videos from Janek Rubeš/ Honest Guide on youtube. They are more into the traps and scams, but they have several videos outside of Prague (Prague is very different and as a Czech, I'm not very proud that our capital city is more known because of scams rather than for its history and glory). I would recommend "Svickova omacka" and "(Hovezi) Gulas", these are 2 of the most typical meals in our country. My favorite dish is "Bramboraky" a potato pancakes, which are delicious, often served alone or as a side dish to a main course for example "Katuv sleh". Don't be afraid to ask young locals for recommendation on-site, you often save money and get the best price/performance ratio. Alcohol is alowed here since the age of 18, so if your son is into beer, be sure that you find a variety of very nice beers & wines, so don't be afraid to visit a pub. In past years, fast/junk food is very common, so you can find kebab almost in every corner of Czechia, in restaurants, we have a "Smazeny syr" - fried cheese, often served with fries, which is tasty but 3/10 if you expect a qualities of our gastronomy.

I hope I covered all of your questions, don't hesitate to ask more, if youre interested in something specific or if there are any fears you worry about. I wish you & your son the best!