Reddit mentions of Words Their Way Word Sorts for Syllables and Affixes Spellers (2nd Edition)

We use a program called Fundations at my school, but a program that might work better, since he can move at his own pace is Words Their Way. They have different levels of difficulty depending on what students know and are able to do. I would recommend starting with this book. It's the lowest level, and will help your brother with phonemic awareness (the building blocks of decoding words and is probably the reason your brother is having a hard time putting syllables together). Then you can move on to higher levels, like this. The program is meant to be a spelling program, but I find that if I pair it with a lot of independent reading and reading with an adult where I can point out examples of the phonics skills they've encountered, then they can learn a lot about decoding from it.

On a side note, one of the major problems that struggling readers run into at a young age is that people spend so much time focusing on their decoding that they don't think about how well the student is comprehending. Use a variety of books with your brother -stuff he can decode just fine, stuff he needs help with, and stuff he can't read at all, but that you can read aloud to him. With these last books, spend a lot of time discussing what he's reading as well. Otherwise you run the risk that he's going to continue to struggle when he gets older because he values decoding the words over understanding the content.