I am looking at getting into mountain biking and have been looking around the local bike shops. I have not mountain bikde before but would like to start doing it with my young son (9yo) on some local trails. I am guessing I might ride 2-3 times a month, so not a lot. I don't want to overspend my needs given that I am only a beginner and won't be riding "hard" or a lot. The LBS has a 2013 Mamba in my size that they are closing out for $600. Does this seem like a good deal for my needs or can I get by with something less expensive to serve my purposes? Is this a good deal on "last year's" model Mamba?

My advice, based on personal experience, buy the most bike you can afford. If the Mamba is that bike, buy it, don't look to buy less. There are two reasons for this. Bike manufacturers buy parts in bulk and pass much of that savings onto you when the bike is new. Upgrading later is expensive. It's much better to buy up when new.
The second reason is, Mountain Biking is addictive. Many of us on this forum started out with upper entry level bikes thinking they won't need a new bike for years only to realize they should have bought a better level bike only a few months later. In my case, I bought an 05 Rockhopper in Sept. of 05. By April of 06 I was in a new XC-FSR. Buy the Mamba, you won't regret it later. Actually, there's a good chance later, you'll wish you spent more. I'm sure others here will add their thoughts on this.

Thanks for the advice. It makes sense. So at the price point, is the Mamba a good choice? Is $600 for a new 2013 Mamba a good deal?

A friend of mine got a 2012 demo Mamba last year for 700. So, yes, I believe that 600 is a great deal. I would suggest doing a few test rides to see if the bike feels right for you, though.

Edit: forgot to mention; my friend is a beginner who only gets out a couple times a month and he absolutely loves the Mamba. Its got solid components, a fork that does what needs doing, and, most importantly, the bike just looks cool.

Bought the mamba and took it out for my first trail ride today. Loved it. I walked some if the more difficult parts, but all in all it was fun. I need to upgrade pedals though. My feet slipped off several times. What flats are recommended?

go with cleats. many are hesitant as they think you are locked in. you are, until you don't want to be. i use cheapo SPDs set loosely. they never come out when i don't want them too and always (pretty much) come out when i do want them to. can't imagine going back to platforms.

Try these bear claw type pedals if you want to stay with flats.
https://www.ebay.com/itm/NEW-VP-990-M...item58a63b8e66
If you go with clipless, try them out and practice unclipping on a flat trail like a rails to trails.
Whatever you do, Don't try straps, AKA traps/cages.