My son was the same way at that age, now he is 10 and loves all animals. He is so gentle and kind with them. We got him a dog when he was 3 and we made him responsible for easy chores for the dog. Have him help with water and feeding. Teach him that he can not feed the dog his food only dog food. Let him help brush and wash the dog. If he helps with these things he can give the dogs treats. He is just curious and wants the dogs attention because he wants to play. Show him the correct way to play with the dog (throwing a ball or squeaky toy) My son used to pull the dogs ears and tail. Every time he did that. I would take his hand and show him how to nicely pet the dog as I said over and over "nice, nice". He started to understand and I just had to say "nice" when I saw him getting rough and he would gently pet the dog. It takes a long time, and hard work to do it consistently, but it did eventually work for me.