I am NOT an expert, but I believe the biggest construction problem when dealing with heavy clay is excess moisture.
It would be possible for us to do that type of project right now in Kansas because there is only a few inches of frozen ground and our groundwater levels are very low. February is usually a dry month in Kansas, so that also helps.
However, in central NY, your maximum frost line should be around 50-60 inches! It won't be quite that deep at the start of February, but it should be close.
I can't imagine how much extra work will be required to excavate under those conditions. Further, you cannot properly compact fine-grained material when it also contains cobble-sized chunks of frozen material that remain after the contractor ripped with a dozer or dug it with an excavator!
I think you should push him back to spring, and ask him to schedule in the sweet spot where the ground has thawed and you don't have spring rains in the forecast in the time period expected for him to finish the pond. (He may want to work now to avoid dealing with wet clay due to spring rains, but I don't see how the contractor can alleviate the frozen soil problem.)
Good luck!
