You really haven't said what he is doing wrong, which makes it kinda hard to figure out if the punishment is excessive or fits the crime.
The puppy might have some relationship to what is going on, but I would be a bit amazed if that was what was effecting his attitude at school.
To me, your most important statement was, "He has stopped interacting in class". I think you need to talk with his teacher and try and get her feelings about what is happening. It very well could be something (as other people have suggested) is going on at school.
You said you wre worried you wre punishing him too much my answer is it sounds by your post as if you are , the whole post is about punishments from spankings,yelling, to long time outs and taking away all toys,if your husband is doing the same thats two of you punishing, could you try the other tack, focus on his positive side and praise him when you see him doing something right. He sounds upset time to try positive reinforcement instead of negative ,
Maybe have the puppy visit with a family member for a week or two and see if his behavior does not start to change back. Maybe he thinks the puppy is getting the attention he use to get. Maybe telling him the puppy has to go unless things change?
He is a big boy now...mine is 6 also. Nip it now. That is what they express at school. Can only use advice and know you have tried. Nip it now,keep trying new things.
Staying on the same page is important and hard. But even if you fall from the page from time to time, you will learn to and it will be easier. Its always about learing.
Action and immediate consequence. If he's told to do something and walks away, there needs to be an immediate consequence. No questions asked. And consequences don't always have to be directed at misbehavior. As in: Little Billy cleaned his room without being prompted and consequently was rewarded with an extra dessert. I have a 6 year old son myself and am a fervent advocate of incentives! Not bribery, but something to work towards. And I can't stress this enough.....CONSISTENCY! You and your old man need to be on the same page, which is a challenge in itself (speaking from experience).
This is all conjecture of course, because no one really has all the answers. Good luck!
Does your son play video games? The only reason I ask is that when my son plays video games for more than an hour a day he gets very mean and does not listen to anything that we say.
It also sounds to me like maybe someone at school is bullying him. He may be under a lot of pressure at school. Sometimes at the age of 6 (from watching my son who is 11) a child is under a lot of pressure to achieve so much. When we all were in school at the age of 6, we were coloring and learning abc's. Today, children at 6 are learning how to do word problems, reading at a level that is required by the school system, and doing writing journals. That can be a lot of pressure for a little guy. When my boy was 6, he had a bully in school that was always teasing and picking on him and he would be very defiant with us. We did not find out what was going on until 2 months after the incident. The school will not always tell you when their is a bully at school. Also, anxiety from being picked out of a class from the teacher to tell him he is doing something wrong can also cause this behavior.
Hope that this helps
Have you sat down and talked to him asking him if something is wrong and what it might be?
What kind of trouble did he have with talking, micnic? Like, slurred words?