My oldest turned 5 in December and I would say he is currently doing the types of problems that you have written above. My son is not even in K until this fall. Luckily he is in Montessori right now and although they allow some advanced math, he does much more math on his own at home. I can also relate to the asking for math problems all the time.
My son has been tested and has scores >99.9% in IQ and in achievement in Broad Math. IMO, based on your descriptions your son would definitely score in the HG+ range if testing.
You are indeed correct that this is more than 3rd grade math and in some instances material covered in much later grades.
I think that testing would be helpful to see what kind of accommodations are needed for your son. Is his current school providing more advanced math? Is he happy at school? How is his reading and language skills? Can you request testing from the school?
