That depends on how old your boyfriend's brother's child is. If the child is still young, then give him some fun toys. Children like them! For example, a color-changing sun jar. If you put it in the sun during the day, it will automatically collect sunlight, and it will automatically glow at night. Such a gift will definitely make the child like it very much!
If the child has reached school age, then give him some educational books or toys. Things that help brain development will definitely please the child's parents. Who doesn't want their children to be smart?
Of course, giving gifts is to celebrate the child's birthday and make the child happy, so it's best not to give school supplies. His parents will buy these, and if you buy them, it may cause waste, and it will not reflect your intentions.
Finally, I recommend you a birthday gift, a family of three real dolls. You can use photos of your boyfriend's brother, sister-in-law, and children to customize them. The effect is very realistic. It is a very personalized gift, and it can be preserved forever. It is very commemorative. When the child grows up and sees this gift, he will know what he looked like when he was a child. It's very good!