No specifics are given in the film or the film's novelisation regarding the food situation. We know from the dialogue that he used the cryo-sleep pods to hibernate for a couple of long periods (10+ years apiece?) in addition to spending several years on his own.
BRAND: Why didn’t you sleep?
ROMILLY: I did a couple of stretches. But I stopped believing you were coming back, and something seems wrong about dreaming your life
away. I learned what I could from studying the black hole, but I
couldn’t send anything to your father. We’ve been receiving, but
nothing gets out.
Interstellar: Original Script
Assuming he was fully awake for three entire years, he'd have needed at around 1500 calories a day which equates to around 700g per day of astronaut food. This figure might be even even less given that food-science has evidently improved in the future. Romilly would have needed a grand total of 700(ish) kilos of food, an amount that could easily have been stored in one of the two large habitat modules.
The Habitat Modules also contain the stores of consumables required
for the mission. Water and air recycling units as well as primary life
support are housed in the Habitat Modules below the floor of the crew
http://endurance.interstellarmovie.net/ [link obsolete]
He might also have been cutting into the "surface mission stores", which presumably also contain some food.
The lower sections of these pods contain surface mission stores and
can be detached and ferried down to a planet’s surface using the
For the record, 700 kilogrammes of Skittles looks like this. If the food of the future was in pill form, there would have been enough space in the hab module for approximately 100-150 times as much as the amount below.