Thank you for your question..
Please advise what your son did/does for a living
You need information on hand relating to
- interest received,
- dividends from shareholdings if any,
- earnings from employment P60s
- profits from trade if self employed
- any other income
- contributions into a private pension scheme if any
for the tax years ended 5 Apr 2013, 5 Apr 2014 and 5 Apr 2015 respectively.
Once you have the information, I will be able to help you.