How long has he been soil sampling your fields?  Soil Sampling every two  years is good to get a history started. Once you have a history, and can see what your trends are, you can probably stretch it out a bit.

In PA, for nutrient management Plans, we are required to have soil samples current within 3 years. So, you should look to determine if you have a state-mandated nutrient management plan; if so, what are those regulations.

If you are growing just corn and soybeans, and if you are using just commercial fertilizer, you may not see much change over 2 years.

IF you have forage crops in your rotation - particularly alfalfa and grasses - I would stick with the 2-year sampling.  Those crops can pull a lot of potassium out of the soil, and that is the nutrient that will change the most in those crops.