Hi! We have done all of the character dining for boys at DL and CA and also Medieval Times (but not Pirates Adventure). I would say, its whatever your kiddo is into....if he really likes knights, jousting, horses, etc. he might really like Medieval Times, if not, I would go with a Disney character dining experience. Its right there, and not as expensive.
I would say for a typical 5 year old boy, the character dining would be great. We really like the Minnie and Friends breakfast at the Plaza, especially if you can get an early reservation that will allow you to enter the park before it opens. It is absolutely magical to get some photos on Main Street, and especially in front of the castle with no one behind you! A good variety of characters can be found there. Once we had a memorable photo with Captain Hook swordfighting my son when he was dressed as Peter Pan.

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We also liked Storytellers Cafe breakfast with Chip n Dale, and the Lilo and Stitch breakfast at the Paradise Pier hotel...it is now changed to a Surfs Up breakfast with Mickey and friends, no more Lilo/Stitch. Both of those had fun "parades" around the restaurant every so often, where the kids could join in, play instruments, etc. Its a bit of a walk to Paradise Pier, but the Grand Californian (Storytellers Cafe) is just beside California Adventure so not too bad.
I can't recommend Goofys Kitchen for dinner. It was expensive and the food was NOT good, IMO. Especially the desserts. They looked like this incredible variety of yumminess, and every single one tasted the same: bland and yuk.