Hi gummie,
I am on total breastfeeding too and my boy is on a flexible 2.5 to 3 hr routine in the day, and 4-5 hours at nite. The time per feeding varies from 15min to 30 mins, depending on how hungry he is.
How many weeks is your baby? I would think every 1.5 hr for feeding is a little too frequent, so I can imagine your exhaustion. Is he getting enough wet diapers, and putting on weight? Is his suck strong, and does he suck continuously for 15 mins and you can hear him swallow? If so, he should be able to last for at least 2-2.5 hours. And also does he wake up for feedings at nite? If he sleeps for many hours straight at nite without feeding, he might be making up for the lost calories in the daytime. Check how much milk you have by pumping. A baby can suck about 1oz more than what the pump can pump out. Get help from the lactation consultants if you can.
If he's getting enough milk, he could be doing comfort nursing, i.e. wants to suckle. You can give him a pacifier to meet his sucking needs (if you are not against it). Or try to delay his feeding times gradually, maybe 10 minutes each time. Don't feed the minute he cries, stretch it out and see how he does. Too frequent feeding might mean he is not getting the rich hindmilk that is calorie-laden. He might only get the foremilk which is more thirst-quenching and that's why he never seem full. The feeding times might eat into his time for sleep and he might be cranky as he is over-tired. I realised that if my baby is over-tired, it is super super hard to get him to calm down and sleep.