I have done bday meals at CM,wispering canyon,park fare,cinderella's castle,crystal palace,phana's, and boma.
All were wonderful!
Does your son love the characters? If so I highly suggest (and agree with abouve poster) to go with chef mickey's! If they don't matter and can take some serious joking..wispering canyon is a blast (if it hasn't changed their bday shtick)
Why not do both? CM for breakfast and WC for dinner? or on 2 different days?
I think if it is a bday trip..celebrate the entire time!