Baby and constipation

Shawna • i`m a FTM. Had Kashton by c-section on December 16th 2015. He's my everything💙
My son is 6 weeks old he had been constipated a few days and I spoke to a nurse at WIC who informed me soy formula can constipate babies. She suggested enfamil newborn and he's adjusted well but is still constipated. He pushes until he's red but no luck. It's been 4 days. Suggestions?