As many have mentioned before me, please don't leave your EpiPens in the car. In an emergency you won't be able to get into the car fast enough. EpiPens really need to be kept with your son, attached to his person some way.
I know several kids who like the KozyEpi carriers because they come in fun designs and can be worn as either an over the shoulder pouch or around the waist. They come in various sizes to hold EpiPens, Allerject/AuviQ, and inhalers. I prefer the SPIbelt because it's smaller, fits tight to my body, and stretches to hold just about anything I put in there (2 EpiPens + inhaler + phone + key + money). For climate control, we like the Frio wallet. For water activities, Otter boxes and Aquapac pouches work extremely well as long as you inspect the seal every time they close.