Enfamil formula help

LO is 6 weeks and exclusively formula fed. I have been feeding him Enfamil Infant since birth and lately he’s been straining to poop really bad. His poop frequency has changed as well. Took him to the doc this morning for straining and constipation concerns. Pediatrician suggested switching him to a formula without iron....... Is there a formula with lower iron or a formula that will help LO poop without straining. I always massage his belly and bicycle his legs but he still strains a lot. Any suggestions?