There are a few more on the market these days than a year when we were still in the process of transitioning to digital television. I would suggest the following handheld TV's if you want to watch local TVs. Please be aware that your son won't be able to watch any channels with one of these while travelling at a speed higher than 5 MPH.
Amazon.com: Haier HLT71 7-Inch Handheld LCD TV: Electronics - Li-polymer battery rechargeable battery is easy to use and offers up to 2.5-hours of use.
Amazon.com: Viore PLC7V95 7-Inch Handheld LCD TV with Built-in Tuner: Electronics - This one is nice with a USB and SD card input. Has Li-battery rechargeable batteries too.
Amazon.com: Tivax HiRez7 Portable 7-Inch Digital Widescreen TV: Electronics - Li-ion batteries rechargeable batteries but only last up to 1.5-hours. Also comes with a cord for connecting to your car. There is a docking station sold seperately for this unit,
Amazon.com: Viore PLC7V95 7-Inch Handheld LCD TV with Built-in Tuner: Electronics
Now, if your son wants to watch TV while you're in route and travelling above speeds of 5+ MPH, then you may want to consider one of these:
Amazon.com: FLO TV Personal Television PTV 350: Electronics You'll get a few more channels with a FLO TV, but the service costs $15.99 monthly. The battery is rated for 5 hours of use though. There are a few cable networks on FLO TV like ESPN. Other channels if available in your area: ABC Mobile, Adult Swim Mobile, CBS Mobile, CNBC, Comedy Central, Disney Channel, ESPN Mobile TV, FLO TV Channel, Fox News Channel, MSNBC, MTV, NBC 2Go and Nickelodeon.