Puppies do nibble! Our Shiva nibbles our ears, and at her size it's not easy to deflect her attentions (lol). Part of that nibbling behavior is just how dogs interact with other pack members, to show affection or initiate play. I think they also think it's great fun to make us react.
You've got your hands full, don't you? I have a dear friend whose youngest son has hydrocephalus, along with all the special obstacles that can accompany the condition. Regardless of how difficult things get for him, he is absolutely the most charming, delightful young man you could ever imagine. I love listening to what he has to say.
Okay, this next story actually has a point, so bear with me:
My sister recently acquired three young half-children (sounds much better than step-kids, doesn't it?). The middle child kept doing things to annoy the family's Jack Russell, and the same type of dominance behavior was going on, along with one biting incident. My sis, her husband and all three kids love the Jack Russell, and he really is a great little dog, but the behavior was causing problems and she has missed having a German Shepherd like we had while growing up. She solved both dilemmas by going to a German Shepherd rescue organization and adopting an adult female Shepherd. Their Shepherd, being both a herding dog and a female - and already an adult, has taken on the role of the "nanny" and she has very effectively made the Jack Russell's dominance behavior with the kids a thing of the past, in fact, she won't allow the Jack Russell to be alone with any of the kids!
It's something to think about and discuss with the rest of the family and possibly your vet and pediatrician (regarding your son's learning not to annoy the dog). It's always good to have knowledgeable input to a problem. It doesn't have to be a German Shepherd, it could be any type of larger dog with a working or herding background, calm, tolerant disposition and nurturing personality. The key is being adult, and, to a slightly lesser extent, female, although the male Shepherds I have had all my life have had those characteristics. In your situation, though, especially given that your Spaniel is male, a female would probably be more effective. The right dog can be an immeasurable help and comfort, not to mention a treasured member of the family. But you know that, or you wouldn't be here, now would you?
One other thing: though I've never owned a Spaniel of any kind, I've had several friends who had various types of Spaniels, and the males have all had a decided proclivity to mount legs, much to the chagrin of their owners. Maybe some Spaniel owners out there might give you some good input?