He couldn't join the SAS until until 1966 at the earliest - even if he joined the army at 18!
You could join the Army before age 18 of course in those days, but the youngest age he'd have got onto Selection would have been early 20s, as he'd have to have done three years absolute minimum adult service, more like five in his first/parent regiment - I'm not sure many regulars did Selection before the age of 22, so it'd have been 1970 before he'd be in possession of his beige headgear.
Add to that he's "got" an MC and an MM - now that's very rare, as it implies he was decorated in the ranks, and again as an officer - he's too old to have got an MC in the ranks under the new rules. Then there is "his" DSO to that, which is rarely awarded to anyone below the rank of Major/equivalent, and he's now got a haul of medals that no-one in the Regiment has ever had - fact, and it is so easy to check up that he's a real plonker of the first order.