5 week old with awful constipation- what can I do to help him?

he had horrible gas issues and was changed over to soy formula last week. That has been great in helping with the gas, but now he has awful constipation. This has gone on for almost a week now so, he should be used to the formula by now. What can I do to help him out? The poop is so hard, like clay, and it's like he can't get it all out so there is always some stuck in his . Poor boy. it's pitiful. Please help!