While it is not impossible that your town has some local ordinance requiring fencing (it would be *very* unusual, but not impossible--you can check this easily at the town/city clerk's office), as a general matter, no: there is no state or federal law requiring residential fencing. You are under no obligation to pay for this fence--if he wants the fence upgraded, repaired, replaced, etc. he can pay for it; he has no legal grounds to make you contribute.