I nursed both my kids. I nursed on demand 24/7. day and night.
Infants, go through what is called "cluster feeding" which means they will be hungry every single hour and NEED to feed. And this is normal.
I don't know what you mean by your baby is still feeding every 2.5 hours. That is normal. NO baby feeds at the exact same timing every day and they feed, often.
Thus, you need to nurse according to baby's cues and hunger cues.
My kids as babies, nursed anywhere from EVERY hour, to 2 hours to 3 hours at most. AND they did this for several months. And I would nurse using both breasts each session, and they drank me dry. And they had GINORMOUS appetites.
In an infant, they have Growth-Spurts every 3 weeks. Then at 3 months, 6 months, 9 months, 12 months, 15 months and every 3 months. AND at these times, their appetites increases and they need to feed more frequently.
I don't know what you mean by "marathon feed." An infant and baby nurses all day everyday. Non stop.
I always nursed my kids before naps, and after naps. That is typical.
I don't know what you mean by in between feedings. A feeding is a feeding. My kids as babies did not take 1.5 hours to feed, like you said yours does.
Perhaps, see a Lactation Specialist and they can tell you if he is feeding correctly and/or if he is getting enough intake etc. And they can check his latch.
What should you do about the "in between feedings" and the "marathon feedings?" Well, you nurse him.
Breastfeeding should always be done on demand. Not by a schedule.