I work for a cellphone store so I'll be honest. The only reason I would recommend a 64 gb is if he will download music videos and movies into the iPad. If he's mostly going to use it for apps and music, 32gb is more than enough. 16gb is good but trust me, once you start downloading stuff, gigs start filling up quick. I have a 32gb with many apps and about 500 songs and I still have plenty of space. I definitely recommend the 32gb unless he really won't download anything at all. If web browsing is all he'll do, 16gb is enough.
4g service on a tablet is worthless in my opinion. If you have a smartphone already, you might as well pay for the tethering service on your smartphone and share the internet with your iPad. Wifi is available in many, many places so paying for 4g service is a waste. I work for one of the biggest cellphone companies in the country and I would never pay for 4g service. I have the first generation iPad with 3G built in and I've never used it because I turn on tethering on my phone and share the Internet with the iPad.
If you're going on a trip, just go into your local cellphone store and add tethering service for a few weeks while your away. This way, if your out and about while on your vacation and need Internet on your iPad, just turn on tethering and your iPad will pick up the signal from your cellphone.
An iPad with 4g service will cost an extra $100. I would rather spend that money on a case or AppleCare to protect the iPad in case I dropped it and breaks.
The new iPad 3 will have higher screen resolution and be a little faster. iPad 2 is not slow, but just not as fast. If he's going to be gaming and switching between apps often, I would go for the new one. If web browsing is all he'll do, iPad 2 is more than enough.
If you have any questions about 4g pricing, let me know, I sell these 5 days a week.