so since my sons doctor switched his formula he has only went poop once & it's been like a week now.…

so since my sons doctor switched his formula he has only went poop once & it's been like a week now. she said its normal & it will take time to regulate since we have had to change it twice since he's been born , but is there anything I can do to help him go? I feel so bad cuz he just grunts and pushes till he's as red as can be & cries. I've tried rubbing his back and belly, & doing the leg exercise things & my mom suggested letting him soak in a really warm bath & I tried that but nothing.

advice? tips? ideas? he's onto a month a few days