Hello and welcome to the site. Thank you for your question.
I presume your son has already registered for self assessment as self employed.
If he has not done so, then he should register for self assessment first and obtain a unique tax reference number (UTR) in order to file a tax return.
He can register for it online and the link is here
If has already registered and got a UTR number and he started self employment on/or after 6 Apr 2014 then he would be required to file a tax return for this tax year 2014-15, and it would be due no later than 31 Oct 2015 if he sends a paper return or 31 Jan 2016 for online filing.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.