He's not being a brat but you can still help him to understand boundaries at that age. Yes it's hard work, and time consuming. But I did it with both my kids, and I could say no and they would listen. Not every time but the would most times, and it helps them to understand the boundaries. I feel this for me worked better because if you just suddenly one day say no it's hard for them to understand that. IYKWIM? Praise worked really well for us too. Not to compare children to animals, but I had a good friend once say to me it's no different than puppy training and it does work And you can do it without smacks/screaming and with praise and love