Disadvantage of NRT: It's 60km (40 miles) out of central Tokyo,

Advantages of NRT: There are many affordable transit options, including buses and two train lines, even a slow train for ¥1000 (about US$10), not recommended unless you're on a really low budget. You have the widest choice of flights, airlines, and arrival and departure times.

Disadvantages of HND: The flights tend to arrive and leave either way late at night or in the pre-dawn hours, which is inconvenient for checking in and out of hotels, although flights to Hong Kong may be different. Fewer airlines go there.

Advantages of HND: It's a short trip into central Tokyo on a monorail, subway, or cab. Less hectic in general.

Public transit in Tokyo is so amazing that flying into one airport and out of another is no problem, especially if one has a week to explore the city.

If your son does decide to go to Tokyo, he should look into staying in so-called business hotels, which offer simple (like a college dorm room, so he'll feel right at home) but clean and single safe rooms with private bath, sometimes including a Continental breakfast, for between $60-100 per night. If he eats in mom-and-pop neighborhood restaurants, food costs should be the same as or less than in New York.

But enough of that! If he wants to stop off in Tokyo, he should buy and read a guidebook, since he has plenty of time. There are even eBook versions of some of the major guidebooks.