For the carrier, I'd check to see what the signal and speeds are in your area and make my decision from there.
You might also want to think about getting a WiFi only iPad. It costs almost the same to buy a 32GB WiFi + 4G/LTE iPad than a 64GB WiFi only model, so I would evaluate that. Also, he would have to pay for the 4G service and might not need it. Most places have open WiFi networks and if he has a smartphone, he could just use the HotSpot option to give his iPad internet access.
I'd go for 32GB of storage space because it is in the middle and will meet the average user's needs. 16GB might be too little space, whereas with 64GB, he might not even use half of the storage space.
I used to have a 16GB, which had less than 1GB left of space before I upgraded to a 32GB. The 16GB had my music (little under 1500 songs), around 300 pictures and the rest was filled with apps and files (books mostly). The new iPad (32GB) still has 10GB of space so I don't have to worry if I want to download a few pictures or new apps.