As soon as I read your title, I thought "Her DS is probably 5 months old" because that age is the doldrums of the Ergo carrier. They're too big to froggy their legs, but their hips/pelvis aren't wide enough to comfortably spread out (it doesn't have to do with length/weight but rather pelvic growth). In just two months it will be a totally different story and he'll be great in both front and back carries. The Bjorn, even woth waist strap, will kill your back t that age. Plus, you're so close to the age for back carries (which the Ergo is awesomefor), and you can't do that with the Bjorn.
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)