My DS wasn't prem but was very sleepy and I used to set my alarm to feed him at 3 am for the first week or two, then after one night of him refusing to wake up - he was SUCH a sleepy baby - I thought I'd see what happened if I left him. He slept through the first night, then woke at around 4 am for the next few weeks, then 5 am, and then slept through from about 3 1/2 months.
If you're feeding 3 hourly through the day - 7 am - 10 pm, then you're fitting 6 feeds in a day - he might surprise you and start waking in the night on his own accord, as my DS did. But if he's getting 6 feeds in 24 hrs, he won't starve