interest on late payment

..... a client has today asked me how he could charge interest to his clients for late payment of his invoices. He gives typically 30 days credit but is never paid before 65 days on average. When he tried to charge interest to a large company the reply was "provide us with the calculation of interest" and when he offered a discount for early payment this was usually ignored and if the dicounted figure was paid the company would request a credit note. Early payment is vital to his Cashflow .What do other people do ?? Any advise is very apprciated..
Salvatore NICOTRA