You'll find lots of information about different kinds of savings accounts on Moneysavingexpert.com.
However, if your son is paying tax, it might be sensible for him to consider opening a cash isa so that any interest he receives can be paid tax free. Many of them can be subscribed to on a monthly basis, or putting any amount in on an irregular basis up to the current £3600 annual limit (rising to £5100 limit next April). If he's prepared to commit a regular amount to savings every month, then a regular saver account would also be worth considering because they often pay a slightly higher rate of interest.