I am not experiencing this yet but my son is almost 15 months so it may come soon. I would first change your wording and maybe reaction. If he is laughing at you then no isn't effective any longer maybe you could say quiet hands, we don't hit, quiet body, not okay, not nice, etc. Change your tone too. If you jump up and yell NO then quietly and softly say your new phrase. Also, I am sure you know this, but you cannot laugh. Kids pick up if you are not serious right away. You don't want him to associate hitting with laughter. I would remove him instantly, say your key phrase, and set him in a specific location each time, don't make any eye contact, and make him stand/sit for a specific amount of time (no more then 1 minute though), then debrief about what happened. He may be a bit young for the debriefing part but if you do this EVERY time he hits he will get it in the following months. The key is to be extremely consistent regardless of location and to extinguish the fun activity. When you debrief look at the situation and see why he hit and try to work on those skills by talking or role-playing. Aggressive behavior is always a way to communicate something so you have to figure out what he is trying to communicate. I personally would not ignore it, hitting is a big deal. I will also assume you don't hit him b/c that could add a whole other element. Regardless of which advice and tactic you take as long as you are consistent it will work, but it may take awhile for him to start changing the pattern. Good luck...