In addition to what Maria said... um, you said you have tried talking to him.. .and that his response is always "I know" and then he talks back. So... my thinking is that your son is being talked "at"... NOT talked "with."
Perhaps, talk WITH him.... see what his thoughts are, his feelings, his stresses, his frustrations. Then listen with an open ear, and do NOT judge him or correct him. Let him free talk, openly and with comfort and safety. THIS is what children need. Children NEED to feel validated... that they know they are being "heard" and listened to... and knowing that they can TRUST their Parent in telling them anything, good or bad.
THIS will provide a foundation for later, in his teens, when you will want him to communicate with you, and share his life with you.
Children need to know that they can go to their parents for anything. Encourage communication.... instead of that they "cannot" communicate with you.
Your son is obviously stressed and/or frustrated about lot of things. Provide him with a means and the tools to convey that to you... encourage him... listen to him.... let him know it's okay to be grumpy... but to talk about it. Heck, even adults get grumpy and angry... and all WE want is for the other person for "help" us, right? Not, chastize us or ridicule us or punish us.
For us and our 5 yr. old... she is sassy too... but mostly when she feels we are not there for her or not TRULY listening to her in our busy days. Then, when we actually sit WITH her and let her chat away... she then feels more connected to us, safer, secure, and "happy." We always let her know that she can come to us for ANYTHING under the sun.... and we will respect her thoughts... and talk about it together. That we are a family and a team. We don't go around punishing her for any little feeling she has that "we" don't like...rather... we try to see what is bothering her, and try to problem solve that, together. Kids need this. Kids are so often just talked "at" and scolded or punished or disregarded.... that constitutes most of their day. Not fun for them.
When/if my girl turns "sassy".... I will sometimes just stop, and ask her "why are you being so sassy... is there a reason you are so unhappy now?" And then I LET HER explain.... and I don't yell or scold her for it. THEN it becomes much clearer and then we can BOTH trouble shoot the issue. Sometimes she is just tired from a long day and fussy, sometimes she says she just needs to wind-down and not be rushed... sometimes she is just feeling grumpy (which is fine... as parents we get grumpy too, and it's not just OUR "privilege" to feel that way), and sometimes it's just she is hungry or sometimes there is no reason at all. But we handle it Together.
Also, he is transitioning to another grade level and school expectations and responsibilities... THIS is stressful for a child. We cannot expect them to just move on without any concern. As a child, I myself was often "nervous" about entering each new grade level... kids have their own kind of stresses too. Remember that. Kids are not exempt from stress or problems. What is real to them, may be nothing to us. But they need attention regardless, and comforting. This helps our girl at least. As a child... they really love to just talk about their "new" class, their classmates, the teacher, their thoughts, how they feel etc. It's all so new and exciting or stressful for them... as a child, that is what I remember it feeling like. And for our girl, she always likes to chat afterschool, just rambling about her day and what she went through big or small. It's important for a child to do this. It helps them to deflate, to organize their feelings, to bounce ideas off of a parent, to "see" what life is all about... to learn that their feelings are normal AND that it is important. My late Dad did that with me... and it REALLY matters to a child... it forms their experiences and their attitude and provides a "center" for them, and helps confidence in POSITIVE ways.
Since you have a boy... I would REALLY work on nurturing his ability to communicate, to express himself, to be in touch with his feelings... to feel "okay" in doing that. It's so very important... and in the years ahead as he grows up. You do NOT want a pent up, frustrated child who is unable to communicate with you, or a child who feels un-safe in doing so. When a child feels "judged" they will retreat and stop talking... and stop leaning on you for help. They need to feel it's okay to tell their parents anything. That someone is there for them through good or bad.
Also talk with his teacher... see what is going on there.. .or with the other kids, socially etc.
All the best,