What if my patent is found to be used by others during the application process?

Keep letting him use it, because the more he sells, the more tort compensation you will get. If your patent is under trial, you can't get patent protection, only temporary protection. At this time, you can negotiate with it and let it pay the royalties, otherwise you can sue him after the patent is granted. Don't rush to sue him, but secretly collect evidence of infringement and relevant sales data, and then initiate patent litigation when the time is ripe.