Firstly, hardly anyone uses traditional modems these days for Internet access. The vast majority of residential Internet connections in this country are via Broadband, either wired or wireless.
How your son will connect to the Internet will depend on what connectivity is available in his flat. It may already have a Broadband connection either via cable or through an existing landline. Or, it may be that the owner of the building has provided a wireless network for the tenants.
If neither of the above is available, he will almost certainly want to go for mobile Broadband - chances are, he'll get the best deal from the company which supplies his mobile phone...