Well there are a few options, yes you could edge him less and just not get him as close until he learns more self control and you learn to read his ‘signs’ better that he’s about to cum.
But the major thing is that you ruin the orgasm. As soon as he starts to cum you have to stop stimulating him, nothing apart from perhaps holding the base of his cock.
In fact, let’s make a Locktober rule, as this can happen to any of us. If he unintentionally goes over, firstly you HAVE to ruin it, but secondly it’s his fault for not telling you to stop early enough, so, you are to catch it in the palm of your other hand and make him eat it immediately. Maybe that’ll make him say ‘stop’ earlier next time!