Since you are classmates and he treats you very well, I believe you also know his interests and hobbies. You can choose a birthday gift that he likes according to his interests and hobbies.
For example, if your brother loves sports, you can buy him some sports equipment, such as wrist guards, knee pads, etc., or basketballs, footballs, etc.
Another example is that he likes reading or writing, so buy him some books that he likes to read but hasn't bought, or an exquisite pen, etc. These gifts are selected according to the interests and hobbies of your brother, and giving them to him will show your heart.
In fact, it doesn't matter what gift you give your brother. What matters is your heart. If you can make a birthday card for him by hand and write your blessings on it, I think it is better than any gift!