I have three sons, and we have dined as a family a handful of times at Cinderella's Royal Table. I'll admit that while they weren't overly joyed at meeting princesses, they have enjoyed the ambiance and decor of the castle (it isn't dripping in pink, but rather has strong jewel tones and dark wood) as well as the opportunity to actually go inside the castle. The food is quite good, too, so that is always a feature they applaud.
If you do a search on Google Images, you could show your son pictures of the restaurant to earn his approval beforehand. You may also suggest that this special meal is for his sisters and then let him select a restaurant of his choice for his special meal. There are plenty of options for hungry boys that are princess-free zones:
ESPN Club,
Sci-Fi Dine-In Theater,
T-Rex,
50's Prime-Time Cafe,
Whispering Canyon Cafe, and
Boma.
I had a friend who faced a similar situation, and they opted to split up. Mom and daughter had lunch at the Castle while dad and son went elsewhere. Their compromise kept everyone happy.
By the way, you do want to make sure and make reservations for this popular restaurant as soon as you can. All guests can make reservations 180 days out; and if you are a Disney resort guest, you will be able to make 10 days worth of reservations at that time.
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