The first thing your son should do is to send a formal letter (by recorded delivery) demanding payment of the money owed to him. The letter should be dated and clearly show the name and address of the publican, together with your son's own name and address. (Your son should, of course, keep a copy of the letter, together with his Post Office receipt). It should (roughly) be worded as follows:
"To Alfred Jones
I write to DEMAND payment of the sum of £xx owed to me in respect of wages for three days work (on 11th, 12th and 13th December 2009).
TAKE NOTICE that if the aforementioned sum is not paid to me within FOURTEEN DAYS I will initiate court action, seeking payment of BOTH the aforementioned sum and my LEGAL COSTS.
Signed: Teddy Bon Jnr."
If the guy doesn't pay up, your son should use Money Claim Online to seek payment:
http://tinyurl.com/5kwx6
Chris