Feeding/scheduling

My son is a month and a half old. He was 6 lbs 13.5 oz when he was born and probably weighs about 9 lbs now (his last check up was at 2 weeks, weighing 7 lbs 2 oz..his 2 month check up is around the corner). He has pretty much set his own schedule - 4 oz of formula every 4 hours.

I recently just started going back to work. I work 2nd shift. 3 to 1130 & usually I get home around midnight. I feed my son when I get home and again at 4 AM and so on and so forth.

Sometimes when I come home, he is still sleeping but I'll wake him to feed. I've been very strict about keeping him on this schedule, because even though I'm exhausted, it's working and I'm scared to tamper with that. But, would it be ok to let him sleep longer? How well does/did your LO sleep at night around this age? And what does/did your schedule look like? Am I feeding my soon too little or too much, or not often enough? Should he be sleeping longer stretches at night?

Just paranoid that I'm doing something wrong because my MIL is always criticizing my parenting & as soon as I think I'm getting a hang of this new mom thing, she makes me 2nd guess myself :(