True "tone deafness" is very rare, so I don't think there's any strong connection between dyslexia (which is not rare) and true tone deafness.
Some people have good "ears" for music - they can hear whether or not they are in tune. This is very important in singing, because there is a great range of pitches we can sing, and if the pitch is off slightly, it can sound really out-of-tune. Violin and other string instruments also can have a great variation in pitch, depending on the exact finger placement - 1 mm too high or too low can make the note WAY out of tune.
If your son doesn't have a good "ear", it doesn't mean he can't play an instrument. Maybe singing isn't his thing (although some people can be trained to match pitch), and string instruments might be tough for the same reason. I would suggest piano, which is tuned professionally and remains in tune with itself. And there's always percussion...
