Yep, they do this about this age... from about 4 years old and up.
My son is 4... and he has been in this 'phase' since he turned 4 years old. He was previously... such an Angel with a mouth of sweet honey. And he can now.... debate like an Attorney... just like your son and back-talks and can be very....sassy.
Next, will be them doing this talking back as a Tween and Teen.
So this is a 'window' into what it will be like later... maybe.
Nip it in the bud.
Next, the book "Have A New Kid By Friday" by Leman, has great tips. Not derogatory. Practical applicable tips... for engaging the child in respectful.... behavior and reactions.
For me... I don't debate or negotiate with my kids... when they are back talking or acting like that. I just say NO... and then walk away. They may protest or scream/tantrum... but I hold my ground. I say NO... that they are being disrespectful... to their MOMMY.... and I will not put up with it. I tell them I can either be "nice" or "mean and strict" and it is THEIR choice. Then I walk away. They get the point.
Oh, once my son wanted chocolate. I said, sure you had a nice lunch and ate well, you can have 1 piece only. He understood. He said okay. He ate the chocolate. Then he wanted another one and brought me the box. I said NO. He then started to argue about it quite madly... I told him I said only 1 piece and that's all... and he understood. But now, he is having a tantrum about it. So I told him, if he does not stop I will throw it away. He didn't believe me and kept arguing about it and getting so sassy about it. So, I got the box, and then threw it away... right there. He was aghast... he did not believe I would do it. But I did. It stopped him right in his tracks. I then explained, that now... no one... can eat chocolate.... not even me... and that was unfortunate... but that screaming at me for a treat... is not going to work.
all the best,
Susan