It is not possible to upgrade an iPad to cell capability internally. The only way to get cell capability in a WiFi-only iPad is to either get a MiFi device - for AT&T see:
or tether the iPad through a cellphone with tethering capabilities. If you think your boyfriend might want cell capabilities in the iPad at some future time, you'll want to purchase the WiFi+Cellular model now. Cell plans are all month-to-month in US-sold iPads - there are no long-term contracts - so he can sign up at any time, and cancel at any time.