[IggleSniggle]

Fascinating. I also had an allergy to “orange” colored foods as a kid, but it looked like asthma, not a stutter. However, the very worst was orange soda, which was hospitalizing. I wonder if there was any relation to your son’s stutter.

[implements]

Possibly https://en.wikipedia.org/wiki/Tartrazine - a common food colouring with a history of possible adverse effects.