The Jumbo is the largest bodied of the acoustic guitars, but size is not its only distinctive feature. The Jumbo body is quite deep and for a small person it can be uncomfortable to hold. Having said that, though, one of our smaller forum members plays a Jumbo...says he liked the sound better. You might think that being the largest of the acoustic guitars it would be louder, but it is not significantly so. A distinctive feature of the jumbo is a huge lower bout.
I own three Mini Jumbo models and having grown tired of Dreadnought size guitars I find Washburn's Mini Jumbo models quite comfortable. The body shape mimics the Jumbo with the larger lower bout, although it is smaller overall. It is also shallower than a Dread or a Jumbo. IMHO the Mini Jumbo may be the best choice for a new learner wanting to play a steel string guitar.
I have no experience with the Petite Jumbo, sorry I can't help you there.
Size CAN be a huge factor. If your son is not at his full growth size yet I would say give an acoustic Mini Jumbo a try.
You mentioned the Festival Series...and I notice that a search for Mini Jumbo on Washburn's website lands you in the Festival Series section. You need to be aware that the Festival Series guitars sound VERY weak/thin when played acoustically...but they really shine when played through an amp or PA system. I would NOT recommend a Festival series for a beginner/learner guitar.
Have you considered a classical guitar? They are always my recommendation for a beginner. They are small bodied enough for almost the youngest of new learners, but if your son is quite young classical guitars are available in 3/4 size, too. The biggest barrier for a new learner with a steel string guitar is the pain caused by the steel strings. The two higher pitched strings are really just pieces of wire and can hurt tender fingertips quite easily. The foremost reason most beginners will not keep up with the daily practice necessary to learn to play well is the pain from the steel strings.
If I may make a recommendation, why not try to locate one of Washburn's acoustic-only Mini Jumbo models on the Internet. My first recommendation would be the WMJ21S because of the tone wood selection, but if you prefer a more "woody" sound to your guitars there is a WMJ11S made with different tone woods. I own three of the WMJ line...two WMJ21S models and also one of the WMJ11S models...and also gave away a WMJ11S to a promising young musician who was playing junk.
Long response, I know...please feel free to ask more questions.