My 5 month old constipated... please help?

Hi, my 5 month old is constipated. Since this morning, he has left "skid marks" in his diaper, nothing more. Throughout the day he cries and I would change his diaper and I can see that he's trying to push out the poo, but it looks too big to come out of his ..... so nothing except little bits have come out all day. I recently changed over to Soy formula. How can I relieve him of his constipation? And how is it caused? Please help, thank you all so much.