Everything InmateAid sends is print. Physical magazines printed by the publisher and mailed directly from the publisher to your nephew at the facility. That is the only format that works for inmates and the only format InmateAid offers.
Digital magazines are not an option for this purpose and would not work even if they were available. Inmates do not have personal internet access or devices that can receive digital subscriptions. Everything that reaches them comes through the physical mail system, which is why print is the only viable format.
The direct-from-publisher requirement is also important to understand. Most correctional facilities require that magazines come directly from the publisher or an authorized distributor rather than being mailed from a private address. This policy exists because facilities have no way to verify that a magazine mailed from an individual has not been tampered with. A subscription through InmateAid satisfies that requirement automatically because the publisher handles the mailing, not you personally.
Puzzle books are a separate category from magazine subscriptions and are worth confirming with InmateAid directly. Email aid@inmateaid.com and ask what puzzle book options are available for your nephew's specific facility. Facility policies on books vary more than magazine policies, and having someone confirm what is accepted before you order saves you from sending something that gets turned away at the mailroom.
For the magazine subscriptions, browse the available titles at InmateAid, pick something your nephew would actually enjoy reading, and the publisher handles the rest.