Holiday entitlement is calculated on an annual basis and (subject to providing reasonable notice) an employer is free to determine when those holidays are taken. So, assuming that the employer started the calculation in November, the employer would be perfectly entitled to deny your son any holidays at all until the end of the working year (i.e. around September and October). While it's obviously unlikely that the employer would actually do that, no rules will have been broken if your son hasn't yet been given any holiday entitlement and isn't given any for several months to come.
Your son's statutory annual holiday entitlement is 5.6 times the number of days he works each week (capped at 28 days). So assuming that he works 5 (or 6) days per week he's entitled to 28 days paid holiday per year. It needs to be remembered that all 'enforced' holidays (e.g. when the business is closed, such as on Christmas Day) are included within that figure.
To complicate matters further, we don't know how the employer calculates 'holiday years'. Some employers start the calculation for each employee separately (using the anniversary of the date upon which he/she joined the firm). If that's the case for your son's company, then his 'holiday year' runs from the date he joined the company in 2013 to one day before that date in 2014.
Many other firms though work to a calendar year. If so, your son was entitled to a fraction of 28 days holiday in 2013 (with that fraction being determined by the date that he joined the company) and then 28 full days in 2014.
So, to return to your actual question, there is/was no specific holiday entitlement between November 2013 and April 2014 because it's not a full year. The only time exception would be if your son was to leave employment in April; the employer would then have to allocate a pro rata holiday entitlement to your son for the period that he's been with the company.