Assuming you can get from the US to Poland, getting from Poland to the Czech Republic is a piece of cake. Just have a vehicle pick you up at the airport and drive you to your destination.
I've chosen this alternative when traveling without a pet. Once you factor in security delays and the fact you have transit to/from airports, it's about the same time as flying and quite a bit faster than by train (assuming that would work). It's a totally custom experience, so I'm sure your driver will be more than happy to stop for Lenny to do whatever. On the cost front, if there are two people (you say "we") then the van is likely about the same. If more then the cost is likely less. A bonus is you can go from airport to your in law's doorstep.
Of course if you are flying into Frankfurt or Paris then this won't work. Generally Europeans are more relaxed about dogs in public places so that would be an advantage.
Having said that, not sure about travel restrictions due to COVID. Those might change things. You'd have to Czech (sic).
I'm also assuming you know your dog, so unlike others I'm not overly concerned about the plane trip(s). Even a short trip in a crate has to balanced against weeks of living apart from his pack, which comes with its own trauma.