A healthy baby will not starve themselves. He will wake up when he is hungry and will sleep if he isn't :). At 1.5 mos, my LO was getting up twice at night. Bed at 9, wake at 1 to eat. Sleep until 5-6, eat and then sleep until 8 or 9.
Are you FF or BF? If you are FF multiply his weight by 2 or 2.5. That should be an estimate on how much formula he should consume in 24 hours. My LO is now 4 mos and 15 lbs. He is taking 27 ounces in a 24 hour period which is a few ounces less than 15x2, but he is gaining steadily and is satisfied after his feeding so I am not worried about it.