Psychology is actually one of the most over subscribed courses in this country. A lot of people I've met in my years of working have completed and got brilliant grades at university in this subject, but have ended up in completely unrelated job roles because there is very very little opportunity open to you unless you are willing to take on a Masters or registered with the BPS and continue education. I'm not saying it's a "dead" degree, because it's not, and can lead into incredible things, but it's VERY common and as a consequence, there are far more graduates coming out with very high marks than there are job opportunities.
I think your son has actually made a pretty shrewd move. He can defer his place, or better still, remove his application and keep it on the back burner. He has a decent set of grades there, which he can use whether he's 18, 21 or 30. He won't have to restudy, unless he needs a higher grade. He will automatically be considered, and won't have to go through the conditional/unconditional fight that often stresses kids out.
He can work for a year, or two, or 3 weeks - either way, he's not sitting around doing nothing, and it will give him time to recharge after taking 4 A Levels - it sounds as if he's had a pretty intensive time of things and if I'm honest, I would actually recommend he doesn't go to University - he sounds as if more study/educational environment may tip him over the edge of jacking it all in together.
I get why you are gutted - you've paid money for private tuition and it must be a bit galling to hear him say he now wants to do something unrelated. But this is your son's life - his choice, not yours. If he needs to work for a year before realigning his plans (and btw, most 18 year olds I've met who pick Psychology as a degree, often end up wanting to do something entirely unrelated at 25), then what is wrong with that, really? He's working, he's ambitious. He sounds pretty switched on 
PS - My brother is not a graduate and has worked his way up to be store manager. He's on a cracking wage - he started when he bombed out of Uni at 18 because he wasn't ready. He's now ready to go back to Uni and is going to do Politics, and with the savings he has combined with the perks he's received, he can do it far, far more easily than he could have done at 18 