Congratulations on your brother's coming of age. In our place, adults are all only children. No sister will give gifts when they become adults. Moreover, when I became an adult, even my parents did not give me gifts. I am not as lucky as your brother. If I want to give gifts to adults, I would say belts, watches, shirts, and suits, but I don't think it is appropriate to give gifts to an adult who is in transition.
I think it is good to give some treasured gifts. When your brother grows up and sees this gift, he will be very touched when he remembers your care at that time. For example, you can give a real doll. You can make a doll with the photo of your brother on his birthday as a model and put it at home for permanent preservation. He will remember the gift from you, his sister, and it will also encourage him.
Or give him a music box. Listening to the simple music and recalling the memories of the past is undoubtedly a harbor for his soul and a VIP ward for healing when he is injured.
You can also add some food, such as blessing chocolate. Write the blessing on the chocolate and eat it bite by bite. Keep your blessing in your heart and carry it wherever you go.
I hope my suggestions can give you some inspiration.