Before you think he just isn't interested maybe make sure he is getting enough sleep. Is he eating properly? Getting enough of the right foods? Any chance of being shy at school, scared of the teacher, being bullied perhaps? Could he be stressed or worried about something that is taking more of his mind that school? Could he be having a growth spurt? All of these things can make a change in a kid's school performance.
I'm a mom and my kids went through some of those while having difficulties. Is he open enough to talk to you if anything is bothering him? If
everything is ruled out, could be he just needs a little extra help with homework or like a lot of kids, they just aren't interested in certain
subjects. And I agree some stuff seems irrelevant but I look back now and realize much of it does help with what and how we will be when we grow up.
You can also just give him a hug and tell him that you love him and that if he tries his hardest that's good enough for you.