Baby is pooping less. Straining.

Ash • Married 8.2.2014. 1st Boy 11.24.16. 2nd Boy 8.27.18
I've called the Dr. They said if he has at least one poopy diaper a day that he's fine. He used to poop all day. It'd be weird to change him and not have poop. But over the passed week or so, it's been less and less and now he's almost 24hrs with no poop. He's straining all the time and fussy. He's breastfed. I thought maybe it was dairy so I cut out dairy. He's 3 months old. I'd be fine with what his Dr said if he wasn't straining and fussy. What do I do? They basically won't give me any options.