I read over the statute, however I believe a more useful document would be the copy of the written lease agreement... to see the bit about eviction/notice to remove animals, how many days he has, etc. Since you mentioned that its all in writing then show that to a property/real estate type attorney and to make sure its legally enforceable. If so, then follow the procedure set in the statute, since you can cite it as being deemed a reasonable course of action should a lawsuit arise...
State in the letter you send him that the cattle are now considered to be impounded at the property at which they reside and a per day fee is now being assessed for their care and board. The owner has X # of days before the auction is held (at the site). Put notice of auction in the local paper. Should he not remove them before the auction then he'll be given the $ paid for the cattle minus the per day board fees. Be sure to make the fees plenty high enough! I'd suggest having an attorney's office send the letter.
Good luck! Let us know how it goes... and let me know if you want a partner... a free lease on grazing land would be nice, nevermind being PAID by the landowner to run cattle!
PM me if you have heifer calves for sale in Florida.