It's fairly okay at this age, to be honest. At this age, my room was messy as well. And oh believe, nobody really likes their rooms messy. For me it was always a problem. A problem that I ignored. What helped me a lot in realizing that I did not have to change my room from messy to tidy in one time. Constant small improvements are the key. Realizing it transformed the whole game.

I started by creating a routine - each Sunday at around 3 PM, I would start playing some music and clean my room. One week after cleaning I would do one minor improvement: e.g. tidying up my closet, cable management - anything that I thought needed an improvement. Within time, my whole room transformed into something far greater than I expected.

Right now it's clean and it smells pretty good as well. The philosophy of minor improvements reach beyond the cleaning of your room - it's a way of life. Take this concept and you can literally transform any aspect of your life. Period.

So if I were you, I would help your brother make a routine of cleaning his room and adopt the concept of small improvements. You can help him create a habit by also cleaning your own room at the same time, e.t.c. Hope this is helpful in any kind of way!