Talk with him and try to figure out what might help with this. Would a separate small notebook for writing down homework assignments help? The problem could be that he doesn’t write down the assignments, or it could be that he can’t find where he wrote down the assignments when he gets home.
When you grow up, they (college professors, bosses, people who send you bills, etc) often don’t care how you remember to do something, they only care that it gets done. It’s a good skill to be able to figure out alternative ways to do something that you find the standard way hard to do.
In elementary school, they do more hand-holding, more telling you the way to do stuff like remember to do homework. They’ll tell you stuff like whether you should use a spiral notebook or a three-ring binder. In middle school and more so later on in high school, they do less of that. That’s a two-edged sword. It’s really helpful if the way they taught you to do stuff in elementary school is difficult for you, because it means you can find a method that works better for you. The downside is that you do have to figure out by yourself a way to do these things. It took some of us a while to realize that we don’t have to do things the way we were told to in elementary school any more, if that way doesn’t work for us.
I’m terrible at keeping a three-ring binder neat. If I have to keep a three-ring binder, it will generally be in no discernible order, and have all kinds of handouts stuck in random places in it (I’m going to punch holes in them, someday). I do better with a spiral notebook or a composition book, where the medium keeps stuff in order for me. I tried for many years to keep three-ring binders, though, because my elementary school teachers insisted that I do so, and I thought I needed to learn to keep a three-ring binder. It never worked. Fortunately, after I graduated high school (yes, some of us are a bit slow on the uptake), I figured out that I was wasting my time and energy trying to keep a three-ring binder organized, and that nobody would really care if I gave up trying. I liked school a lot better, after that.
There’s an even bigger lesson here, one that I didn’t figure out till I got to grad school. It’s better to work with your strengths than to try and work on your weaknesses. Instead of trying to eliminate your weaknesses, acknowledge them and try to find ways to work around them and get done what you need to do. It’s generally a more efficient use of your time and energy to get better at something you’re good at than it is to try to get good at something you’re bad at.
At least, this was the way things were for me. Maybe this has changed, now that the Earth’s crust has cooled and all.
ETA: Something you should be aware of as a parent is that if somebody’s going to get depression, it often hits around puberty. Be on the lookout for signs of that, especially if there are depressed people in your family.