The Baby Boom had as much to do with postwar affluence and the rise of suburbia as anything involving the number of returning WW2 vets. Take away one or the other, or both, and I think you're likely to see a lower birthrate. The parents of the Boomers weren't just the WW2 generation, they were also the children of the Great Depression, many of whom endured a great deal of hardships growing up. In a sense, their desire to settle down and have a family in a comfortable suburban house was a reaction to the circumstances of their own childhoods.
Frankly, I have a hard time coming up with a scenario that doesn't result in a much higher birthrate short of wiping away America's post-war prosperity unless you drastically change the course of the war. While a return to Depression was much feared after the war, in hindsight, it was pretty hard for the US economy to fail when it was the sole undamaged major power on the planet and normal consumer demand, suppressed by rationing during the war, returned with a vengeance.
Even suburbia was pretty much inevitable; there was a big housing shortage in the immediate postwar years and, with vast tracts of undeveloped land and a return to civilian auto production, suburbia was an easier and cheaper solution to the housing problem than large urban building projects.