I need to represent my home state of California! We’re called the Golden State for a reason. 
Southern California is definitely hot, but can get expensive. It’s near the coast, but if you want to be near water, I’d suggest San Diego over Los Angeles.
Local wildlife? I’m not sure about that, but it’s only a 4 hour drive to Yosemite National Park.
Very multi-cultural, I’m sure you can find a lot of art and culture.
I’m not too sure about the non-aggressive and patient part. LA folks, in my experience, can be touchy. I’ve found San Diego to be much more laid back.
Now, if you don’t mind slightly colder weather (a light jacket at most times, no overcoats or anything), the San Francisco/Bay Area is awesome!
Multi-cultural? CHHHECK! Arts and Culture? Phff, short of NYC, I don’t think you’ll find as much art, live music, or cultural attractions as San Francisco. There is so much culture there.
It is also roughly 4 hours away from Yosemite, and only 2 hours away from Lake Tahoe, a great place to ski/snowboard starting mid November. There is plenty of local wildlife in the form of raccoons, deer, foxes, and such..there are regional parks in Berkeley, Oakland, and San Francisco. I’m sure there are a couple more, but those are the ones I’ve personally been to. Golden Gate Park in SF is very nice to visit.
San Francisco has many accomodations, not all are expensive. You can always find accomodation across the bay in Berkeley, Oakland or Emeryville.
Lastly, the people really couldn’t be any nicer, more patient, or helpful. People are straight up friendly in the Bay. Take my word for it. I haven’t heard of anyone disliking the people there, save the occasional homophobe who makes fun of San Francisco’s extensive gay population.
One major thing to consider when coming to visit the states is your mode of transportation. European countries have much better public transportation systems than America. I haven’t had the pleasure of using every public transportation in all of America, but I can say that Los Angeles has pretty bad public transportation, and relatively, it isn’t that safe.
San Francisco has an awesome local transit system (MUNI) and a very good city to city transit (BART), in the East Bay, you also have local transit (I forget their name). You can get anywhere in the Bay Area using a combination of those three systems from morning to late night. It is fairly affordable, except they don’t offer week passes or anything of the sort. At least, not to my knowledge, it has been a while since I’ve been back home.
I would say that is a critical factor you may want to consider, if you will not be renting a car. America is a terrible place to try and manuver without a vehicle or sufficient public transportation.
I have been to New York City with my mom (single mother as well, and actually I think I was around the same age as your son), and while it was on the expensive side, there was plenty of culture and art to see. The people are not the friendliest, but they aren’t jerks either. I see no reason to avoid NYC because of their ‘attitude’. The transportation is also good, it may be packed, but at least there is one. The level of safety is lower than San Francisco, but not terrible. Like any city, you just need to watch where you are, and what time of the day you’re out. If you are in the shady side of town at 3 am, no matter what city you’re in, you may find trouble.
Good luck with your travels, it should be really exciting for you and your son. 
If you choose to visit SF/Bay Area, I can suggest many places for you and your son to visit.
