Overnight Camps in Southern California?

It's going to be very difficult to find a camp for a 9 year old that is only for 4 nights. A week would be the minimum in most cases. Look into YMCA type camps, or Boy Scouts if you can support them (not everyone can). Specifically ask if they have an "Intro to Camping" type of program for first-timers. It's also very late for camp registrations and while you might find a place with a cancellation, you want to look at the ramifications of a camp already being up and running, with kids settled in, and then introducing your child to kids who are already in a routine. It can be difficult to break in later on.

Have you considered taking your child for a camping weekend, perhaps to a place with small cabins vs. tents? There are social activities that just crop up at campgrounds, with kids finding friends and going to the lake or for a hike together. That's a good way to start exposing kids to an outdoor experience with public bathrooms and showers, a disconnect from technology, and meeting up with people from all over. I would consider trying to do that a few times this summer, and spend the next 6 months researching overnight camps to find a good fit for next year. Also google camping expos and shows where different camps have exhibits and info booths. They will all throw their info at you, of course, but it's a good way to think of questions you have and issues to address with your son to get him prepared and to find the type of camp that would suit him best.

We always found camps for our kids based on what their friends were doing, by asking around, talking to other parents. That's where your going to get the best information for YOUR area. Parents with older kids are the best resource because they've usually had more than one kid go through the process and can give you the best advice.
The one my kids went to was VERY popular, run by an outdoor education department out in the gold country. They took a bus up early Monday morning and returned hot, dirty, tired and very happy :-) Friday afternoon. If you were up here in Nor Cal I'd highly recommend it.

My daughter went to Sea World camp in San Diego for three summers when she was younger. Lots of kids traveled in from out of state and she made some great, lasting friendships. The programs were amazing.

If he is interested in a particular sport, start looking into overnight camps within that sport.

Otherwise, our YMCA has a camp in Big Bear. I think you have to sign up in March for the Summer.

I used to work at a YMCA Camp, and as parents are picking up their kids on the last day of camp this summer they are encouraged to register for Summer 2016 (campers registered this summer have first chance). It fills up very fast, so even right now I would imagine there are waiting lists for some weeks.

The YMCA Camps usually have a great reputation.