Although I'm not a stacker myself I've worked alongside loads as a Techie. They do a valuable job and I have to say it probably is one of the most diverse trades out there. There is real scope for travel and once he's in he could always go for some of the more specialist out-of trade roles such as Mountain Rescue or working alongside the security forces in Northern Ireland etc. I'm sure once he's in and in the real RAF the colour blind thing will become a lesser issue. Once in the RAF many problems that bar entry can be dealt with and managed in a more sensible way.
I also believe that stackers look after the fueling of Search & Rescue helicopters when they are operating in the hills away from the main operating base? Sounds exciting to me!
Finally, don't know if this will sell it for him but I joined up from a pretty average family. Since joining up I've learnt to sail and hold a fairly high up skipper's qualification. I've sailed all along the English South coast, the West Coast of Scotland, Denmark, Germany, Sweden, Holland and the North Sea. I've learnt to ski and I'm pretty good at that. I ride an off-road motorbike competitively and get fantastic support which almost makes it free from the RAF. I've walked up mountains and seen views I never thought existed. Had the chance to fly in various aircraft, including blasting around Salisbury Plain at about 50ft in a Lynx! The list goes on... Tell him to stick with it, the RAF has to be the only job that has the most pressure during the early years, as the time passes it gets less stressful, how cool is that!