KJP, I thought I replied to this last night - must not have hit the submit button lol!
FWIW, we live relatively far north of you, and quite a few folks up here do feel the impact of SAD. Although I don't know of any children who are impacted by it I do know quite a few adults with SAD. I'd second all the recommendations for providing extra light and physical exercise. If there is a time of day that's brightest, be sure to get your child outside when you can during that time of day. Even though they aren't broad spectrum, you might try stringing a few Christmas lights around inside your house wherever your ds spends a lot of time - we find them cheery and folks in our part of the world leave those lights up all winter long until the sun returns
All of that said, the main point I want to emphasize, is to be sure to follow through on the allergy testing. My dd10 has mold allergies - and they can both be seasonal and appear to be seasonal even if they aren't. There are *so* many different types of mold, and an allergist can usually determine through testing which of the different types of mold found in your area are (or aren't) impacting your child. For instance, my dd is allergic to snow mold - it appears in our area in the spring when our snow cover melts. When she was younger, we could see the impact in her behavior. She is also allergic to the type of mold that grows on books and newspapers etc - this isn't a seasonal allergy *but* it can appear seasonal because she's inside school during the winter months where there is more of it. Now that she is older, the symptoms appear less in her behavior because she's more aware of what's up with her body - so for instance, she knows that when she's in the school library and her nose gets sniffly and she feels like coughing, it's the mold and she needs to leave the room. When she was little, she'd just act out because she didn't really understand the connections plus she had no way to "escape".
Mold allergies are tricky to catch - I'm glad your NP was aware of them and recommended seeing an allergist. It might be nothing, but by testing at least you've crossed that question off your list. If it is mold allergies, the symptoms are easily controlled with daily medication during mold season, and it is the type of allergy kids can (if and when a parent wants them to) go through allergy shots for desensitization and the shots work very well for most of the people I know who've gone through them.
Best wishes,
polarbear
