You can pay him voluntarily; or if  you don't, he could try to sue youl you don't have to pay unless he sues and wins. If he sues you, to win, he'd have to show by a "preponderance of the evidence" (that is, that it is more likely than not) that 1) your mower threw the rock(s); (2) that it was your fault it threw the rocks (e.g. you were careless and drove it over gravel)--since you are only liable if it was your fault; and (3) the amount/cost of the damage (he may be exagerating about the damage or the cost to repair). It may be very difficult for him to prove this and win; but unfortuately, that does not stop him from filing the law suit, if he's sufficiently determined, and making you defend yourself. Depending on how much is at stake, and what you are willing to pay for your principals, it may be worthwhile offering him something to avoid the cost, time, aggravation, etc. of a lawsuit.