Go to code.org/learn and start with tutorials there, they are gathered from different sites that teach coding. It is a good introduction to what is available. As bloodyteenagers said Scratch is excellent, you will find it at scratch.mit.edu it's free.
DS (9) started with raspberry pi and the scratch programme. Its hard to self teach at the initial stage though.
Have a look at codeclub.org.uk - it lists places that run coding clubs for kids (usually schools and libraries) and there is also a section where schools can link with volunteers who teach coding and set up their own club.
Schools are getting on board with this where I am (London)...worth talking to your DS's school and seeing if they are interested in setting up a club.
DS attends a weekly club at a library and its been great for him. He has homework each week and is now at a stage where he is confident to start going through his coding books himself and making games independently.
Good luck - its a great skill to have and hours of fun.
Forensic IT is fascinating, and he'll need to be good. I think most of our guys (and they are all men) have had other roles first, like police or military, but they are seriously good technically. So if he stays interested in it, he might need to see if he needs other skills/experience besides just coding - but he's got a few years for that, and starting with coding is a fine plan.
I agree with going to code academy online. There are also loads of tutorials online for specific languages, if he gets interested in a particular one. A Raspberry Pi is also good, but I'd probably start with stuff like Scratch before investing in a Pi. It's worth looking for any code clubs locally.
It might also be worth looking at organisations like Stemnet - they liaise between schools and volunteers, including things like First Lego League, where teams get to program a Lego Mindstorm robot, and they should be able to support schools with no experience. Plus there have been changes in the curriculum this year, and they won't be able to carry on not doing coding.
I'll sit down with DH later and have a look at all of these sites.
Maybe see how he gets on with some of the on line tutorials and then get him a Pi for Easter.
DS wants to join the military or the police in some capacity and work the two things together. (Or if all that fails he said he'll go be the IT guy at daddy's work who tells you to turn everything off then on again!! )
There are also some organisations that do coding summer camps - there was a thread with some links some months back; I have an idea it was just too late to apply for places for the summer just gone, so maybe it was around June/July time.
He should also try to find some code to read. A start would be to decide on an initial language, Basic, pascal or Java for example. Then find a compiler (the program that converts code you write into programs that run). Learning to code is easier in pairs so if he has a friend who is interested as well that would help.