The pedantic answer to your question is simply that nobody needs a passport to travel within the UK.
However, I assume that you're referring to air travel, where some form of photographic ID (e.g. a passport or driving licence) is normally required at check-in. Each airline is free to set their own rules but, if your son is under 16, it's unlikely that he'll require a passport (or any other form of ID). if he's travelling with you (or another adult). For example, the Ryanair website states this:
"Photo-Id for children under 16 years is not required in the following circumstances:
Children travelling with an adult on UK domestic flights and UK-Republic of Ireland-UK routes . . ."
http://www.ryanair.com/site/EN/faqs.php?sect=p id&quest=photoid
Chris