Some work alongside laptops or gaming consoles, such as PlayStations. (They tend to be very expensive and, if they're linked to a laptop, need an expensive, high-spec laptop as well).
Others have their own software systems, with the Oculus Rift being a market leader. However, at around £550 (plus the cost of the software), it's not cheap.
Those designed to work with mobile phones are much cheaper BUT they only work with specific makes and models of phones (so you'd need to know which phone your grandson has got). For example, the Samsung Gear VR (£99 from Samsung but cheaper elsewhere) works with the Galaxy S6, Galaxy S6 Edge, Galaxy S6 Edge+, Samsung Galaxy Note 5, Galaxy S7 and Galaxy S7 Edge. (NB: Those are expensive 'top of the range' phones, so not many teenagers will have one!)
http://www.techradar.com/news/wearables/the-best-vr-headsets-2015-1292087