Just for info here, you will get loads of opposite leaning advice. Some will say it's a strictness issue and you have been too lax, so need watertight boundaries and a hierarchy of consequences for breaking them. Almost like boot camp in the short term with gradual loosening up of the rules once he's behaving in the expected way. The idea behind this is that it is very clear and expectations are set out and small achievable improvements should be rewarded so that there is a clear path which is accessible to him.
Others will say actually rigid boundaries and escalating punishments are causing/exacerbating the issue, and you need to go totally the opposite way, finding a way through to communicate. This is where the PDA, Visible Child and Explosive Child / Ross Greene and Mona Delahooke ideologies come in.
You can't do both because they are completely opposite in ideology, and it will be confusing if you're consuming an entire thread where people are weighing in from both sides assuming that they are all singing from the same hymn sheet.
You should instead pick one and stick to it for at least a trial period of a few months. If you're going to try and do communication/root of behaviour then I would strongly recommend the resources by Ross Greene and Mona Delahooke (I am not personally a fan of Visible Child/Robin Einzig, but some people really like her) because I think they are the best and based on actual solid evidence/experience with challenging behavioural issues.
I don't know good resources for the other option - I'll lay my cards on the table, I don't think being stricter helps, particularly in the teen years/particularly when you're already struggling, I think it exacerbates the issue, and I think that children who are acting out are usually struggling with whatever expectations are already being placed on them, feeling misunderstood and persecuted (even though this is not the adults' intention). It can also be very hard to have full control over teenagers which can make some of the ideas about boundaries and consequences very difficult to actually enact. You don't really have this issue yet at 11, but it's worth bearing in mind for the future. Unless you're dealing with a situation where for whatever reason a child has never actually had clear boundaries and expectations, you probably don't need more of them if they aren't working in the first place, you probably need less.
From what I've seen on MN and other places, it more often tends to be people who don't have experience of children with challenging behaviour who suggest the first option. This is (IMO) because a milder version (ie, standard boundaries, consequences, routines and predictability) have always worked for their children and so they make an assumption that this is what causes good behaviour, so if children are acting up it is because they have not experienced enough of this and that more will help. It's a reasonable enough assumption, but it might not help if you have a child who is reactive for whatever reason to the usual boundaries and consequences. IME, people who recommend the "backing off and looking for communication/root cause" tend to be recommending it because they have experienced dramatic change in a child's behaviour from using these kinds of methods, not just in a theoretical sense or with children who aren't expressing challenging behaviour. IME, persistent challenging behaviour is a sign that something isn't working, and it can often be the typical behavioural methods feeding into this.
I might be wrong. Maybe there are children who have been helped by a stricter approach, and possibly at 11, it would be easier to actually have full control over everything so it might be easier to make this approach work, or you might perhaps recognise that you have been a bit haphazard previously and feel that clearer communication about behaviour might nip things in the bud. I don't mean to suggest that this approach could never work. But whichever way you go, I just thought it might be useful to understand why there are two conflicting ideologies and why a lot of advice you get in your situation will be difficult to reconcile, because people are often coming from one place or the other and tend to feel dismissive of the opposite approach.