The iPad is the best tablet out there, but if he's already into the Android ecosystem with his phone, it would make sense to get an Android tablet. The Google Nexus 7 for $200 is the best one. It doesn't have 4G, but he could use the hotspot feature on his phone. But if he wants 4G speeds on his tablet then the iPad is the way to go.
A LeapPad might be a good choice for such a young age. It's a lot less expensive ($100-$200) and is much more robust. It's also designed with software for little kids. They are very limited though, and not all that modern anymore either. Leo also knows of a lot of kids even as young as 2 or 3 who use iPads very effectively.