The popular medium-sized sherpa carrier weighs 1.63 kgs. Even with a lighter carrier and him losing some weight you are realistically looking at a minimum of 9.5kgs combined weight. I only have experience with LH in Europe, and at least in FRA being almost 20% over the limit they definitely will make you check the dog in the hold.
You might want to ask in the respective airline forums if there's any experience with how strictly the limits on other airlines are enforced but I would not count on leniency.

Apart from booking a seat on a private aircraft (see a few threads below for that), the only ethically sound option*** to get your dog to Poland in the cabin I can think of would involve Skyteam: Alitalia accepts in-cabin pets with a combined pet/carrier weight of 10 kgs (provided they fit in a carrier that's max. 40*20*24 cm) and Delta's policy does not mention a general weight requirement for in-cabin pets but puts the focus on aircraft-dependent size limits instead. Call them at 800-241-4141 for details. Does your dog fit comfortably into a medium Sherpa? Over 8kgs sounds awfully much for a JRT - they usually weigh in less than 6. Try before you fly! The large size sherpa will not be accepted as a carry-on on either airline, and he needs to be comfortable in the carrier - that means he has to be able to lie down and stand up. I've had to argue with agents (LH, again!) about this as well - it took a lengthy discussion, a supervisor and evidence of a ton of prior air travel for them to accept that my dog was fine in her kennel....

Alitalia (AZ) only flies out of JFK to WAW and KRK, though. And Delta doesn't fly to Poland at all - you'd have to fly into FRA or PRG and continue onwards by car or train. There is no way you would get your dog in the cabin on either LOT (8kg limit) or one of the LCCs serving smaller Polish airports like Wizzair or Ryanair who don't accept pets at all