Suzanne- your rule of thumb - if he's waking up happy from his naps, they are probably fine. If he is waking up cranky, then you want to extend. But he is doing EXTREMELY well do be sleeping from 11 - 7 at this age!!! That is REALLY fantastic!!! Especially as he was premature! Congratulations!!!
Just based on the info alone, you're in a good space and I wouldn't change anything even if he has a little A time before his next feed.
Continue paying attention to his cues. He may begin to go a bit longer between feeds. Go ahead and let him. The changes between 3 months and 4 months of age seem to happen very quickly with going longer between feeds and extended A times. And for your little guy it may be more like between 4 & 5 months as he was born early. Since he's sleeping through so early though, it may come quicker for him. Some signs that the routine needs adjusting is shorter naps than your current 1.5, cranky wake ups, night wakings or early wakings.
Also, since you have a nice routine now, log the hours that he is getting in a day - night sleep vs. day sleep. As sleep/eating routine starts to change, the hours of sleep remain similar, just re-arranged. It's always good to know how many daily hours you started with so you have a comparison. E.g. he sleeps 10 hrs at night, 6 hrs during the course of the day now for a total of 16 hrs. This may shift to 11 hrs at night, 5 hrs in the day to remain at a total of 16 hrs.
HTH!