I was want to know if my fetus weighed 4lbs 3oz at 33 weeks 3 days, what should he weigh at 36 weeks 3 days?

Just an estimate: Keep in mind that the estimated fetal weight (efw) on a third trimester ultrasound is just an estimate with a margin of error. Also, fetal weight gain is variable depending on many factors. In general, it is estimated that a fetus is gaining about 1/2 a pound per week at this stage of pregnancy. That would put your fetus at about 5 lbs 11 oz at 36 wks 3 days based on your previous efw.