Or maybe, since the Baby Boom only happened a year after WW2 ended, Operation Unthinkable is actually given the greenlight by the Allies, and thus have to face the meat-grinder that is the Soviet Union. Though keep in mind this depends on how strong the Soviets were at the time. Not to mention, this is a different scenario in and of itself.You're going to need to screw the US in the War, badly. Soviet level badly where the population losses are so bad so constantly that any Baby Boom cannot make up for the dead. That in itself will make for a different economic, social and psychological situation after the War.
You would need to have WW2 completely become life or death for the continental US during the war. The war would need to physically feel desperate. It was in a lot of places but I mean absolutely desperate. You can do whatever "spice" you want for the details: a competent and unnaturally successful Italy, Japan hitting Pearl Harbor worse and with early success in the Pacific, the Germans managing a breakout with the Battle of the Bulge that does not win the war but prolonged it, and probably better terror weapons that hit the US (like the Japanese plan to release plagues really working). And despite all that, you need to walk that fine line of the Axis not winning but not losing for longer and with greater harm to the US.
The emphasis is not fewer Boomers. Though that would happen, it's a red herring. The emphasis would be on a recovering, tired War Generation in a world with a different mood. The parents would be a generation of Survivors and the OTL Boomers would be the children of survivors trying to make something out of a broken humanity that has exhausted itself fighting bullies.
Without the boomers, or the demographic weight they had IOTL, how would this effect the social changes of the 1960s and 1970s? Would America - and the rest of the world - be more conservative?