Stephen Nock was managing director at Bright ideas Studio Ltd, which previously ran Seasons Restaurant in Eastbourne.
In May 2017 a four-year-old boy was served an ice cream with a chocolate hazelnut wafer at the restaurant after his mother told staff he had a nut allergy.
It triggered an allergic reaction, resulting in an eight-hour stay at Eastbourne District General Hospital.
East Sussex Trading Standards found the menu and company allergen folder did not specify the ice cream came with a wafer or that it contained nuts, and staff training records were found to be inadequate.
Nock, 61 of Bermuda Place, Eastbourne, admitted breaching the Food Safety Act and was fined £1,200.
He was also ordered to pay prosecution costs of £959 at a hearing at Lewes Crown Court on 20 July.
The Seasons restaurant is now under new management and has no connection to the former owner.
Since 2014 foodservice businesses have been required by law to list 14 allergens such as nuts, gluten and mustard on menus, or have their presence communicated by staff.
In 2016 ex-restaurant owner Mohammed Zaman was jailed for six years for manslaughter after the death of a customer with a peanut allergy in what was thought to be the first case of its kind.