The concept as a whole is a little cutesy for my taste as well--what's wrong with just typing the words "we're having a boy in October" or whatever? But, that is totally a matter of taste, and I think if your DH finds it meaningful, this would be a good opportunity to make a long-lasting memory--so don't think of it so much as a "gender reveal photo" but maybe more like, a nice project to include DH and DS in the pre-baby excitement.
What about having DH himself in the photo? You could put DH on the couch, next to him DS, and next to him maybe a baby car seat in blue, or in a neutral color but with a sign saying "reserved for Baby Boy Smith" (or whatever the last name will be). Or have the caption be something like "three in a row." Or DH could be wearing a t-shirt that says "I'm the dad," DS's would say, "I'm the big brother," and then in the third spot would be a t-shirt saying "I'm the little brother."