Choir and piano.
You can start piano right now, with the Suzuki method (at Levine Music or or the International School of music, both good schools), but choir will have to wait until second grade, with the Washington Children's Chorus or the Strathmore Children's Chorus, the best kids' choruses in the area.
My inattentive ADHD son with a fine motor disability should have played the piano as therapy for increasing his coordination. He had too much catching up to do in other areas, however, so we did not do so, but he has been singing with Strathmore for years and it's been a wonderful experience. Singing with SCC has led to other singing opportunities - he was selected to sing in the kids' choir of an opera at Wolf Trap, for example.
As a musician, I can tell you that no other instrument really addresses motor ability as well as the piano, with as much musical forgiveness. Press any key, and the correct tone is already there, whereas the strings require rigorous memory to remember exactly where your finger is supposed to press (there are no markers), and the woodwinds need carefully crafted mouth positions. Unlike some other musical instruments, piano encourages good symmetrical posture and the most equal use of both hands.
The Suzuki method is the best for children who need special attention, because its philosophy is that the parent is an integral part of the child's musical development, just as much as the teacher. So the parent is required to go to an introductory workshop to understand the basis of the method, and then to attend each lesson and take notes, so that the parent can guide their child at home. It works very well, however the parent has to make time for this.
Later on, he can switch to the clarinet, which is a beautiful instrument and easier than other woodwinds.