Soy formula and baby having trouble pooping?

I switched my son on soy formula (he has been on it now for 2 weeks or a little more) He was just on regular store brand formula (equivalent to similac regular) and wasn't doing well with it. Spitting up lot's, very cranky, gy, crying lot's almost made me think colic. He also had diarrhea his little poops would literally fly right out of him!! My daughter had problems too and our doctor put her right on soy and she did wonders with it. So since my doctor no longer practices in our area I umed his problem was the same and did the switch. He still spits up loads, but seems more happier. However after his night feeds he his very unsettled and won't go back to sleep. When morning comes and he has to poop he has a lot of trouble to get it out. He strains and cries and brings his legs up. His stools are a little hard, but you have to stimulate his bowls, move his little legs etc to get it out. Then he is much better. He is regular, and goes every morning but with much difficulty. I've tried giving him water to drink and a little corn syrup in one bottle but nothing helps. I can't get into his doctor for a while, so just wondering if anyone has any suggestions.. or any good formulas that are easy to digest. I must add he is a hearty eater and eats anywhere from 5-7oz every few hours and he is one month old.