The Road Traffic (New Drivers) Act 1995 is quite clear, Lynda:
http://www.legislation.gov.uk/ukpga/1995/13/section/1
Section One says this:
Probationary period for newly qualified drivers.
(1)For the purposes of this Act, a person’s probationary period is...the period of two years beginning with the day on which he becomes a qualified driver.
(2)For the purposes of this Act, a person becomes a qualified driver on the first occasion on which he passes—
(a)any test of competence to drive mentioned in paragraph (a) or (c) of section 89(1) of the M1Road Traffic Act 1988;
Note the phrases "...a person’s probationary period is...the period of two years beginning with the day on which he becomes a qualified driver.
And then;
..a person becomes a qualified driver on the first occasion on which he passes—
(a)any test of competence to drive mentioned in paragraph (a) or (c) of section 89(1) of the M1Road Traffic Act 1988;
It is the "first occasion" which is the key phrase. It is only a driver's first test which counts as the start of his probationary period of two years (during which six points sees him revert to provisional status). Once that two years is up he can no longer have his licence revoked under these rules.
So I was not quite correct to say that a driver can only have his licence revoked once. However, to have his licence revoked three times in his first two years your son would have to have (a) firstly accumulated six points (usually two offences) leading to revocation. Then (b) have passed his test again and committed a further offence leading to revocation. Then (c) passed his test again then committed a further offence leading to revocation.. All this would have to have been done within two years of passing his test for the first time.
I have seen a large number of drivers in the position of having their licence revoked for the first time. I cannot recall seeing any driver falling foul of the rules for a second time and cannot imagine a driver having the time to commit enough offences and pass his test twice to have his licence revoked on a third occasion. Apart from anything else a driver in such a position would have accumulated at least twelve points and so be liable to a complete disqualification for a mandatory period of six months. This would eat quite substantially into his two year probationary period.
So I think either (a) you do not have an accurate recollection of events or (b) he was stitched up in some way.