My nephew(age 12) is coming to visit us for the 1st time. We are some "cool" places that we can take him. We don't want to spend a lot of money since we are moving in a few days. We are going to be kinda low on money... Any help please!! 

You can take him to the delmar fair. Right now there is a deal on living social for "the wave waterpark" in vista. It's a small waterpark in a shopping center, but it was fun! They have a deal that you pay $16 for 2 admissions. That's the only thing I can think of being cheap places to go.

Living Social 

The beach is usually a good one thats free. The Childrens Pool in Carlsbad is free and he can see some seals. Its a nice area. Parking can be a pain but its free also. If you havent used your Sea World tickets you can give it a try. They say that kids over 10 need a id but I know a lot who have gone and they nevered the kids for a id.

What kind of things does he like? 

La Jolla Cove is a little drive but the seals are so impressive there.
The Mechanized Museum on base is very small but kids can get in some of the vehicles and it has been a hit with my younger cousins(3 boys, 11, 13, 15). It is on Vandegrift just past the air field. It is free but has a box for donations which I usually put a $5 or something in.
The USS Midway in downtown San Diego is pricey but so, so cool. We live in Oceanside and take the Coaster to downtown San Diego($11 round trip). Military is free but dependents have to pay and I think it is $18 :/
I have heard this placethis place is neat but haven't been there. It says it is a hike but seems neat.
Of course the beach is always fun :)