Yes he does shape change his human form to look more similar, but I speak of the demonic form, which should be unique for all demons as no two look alike.
Also breeding doesn’t often work well. Two demons breeding makes both of them weaker, especially the female, which is why they often abstain from doing so. Demons unlike humans have control over if they will conceive, thus they can have sex as often as they wish.
Most energy comes from the father, so most half demons are from male demons. The child will then in most instances normally at most be a 3 if the parent is powerful. The child can be in rare instances a normal human, but will more often be a human with demonic traits/ abilities, or a rank 1 or 2. Not a significant threat, so most don”t do this. If they could on a regular basis breed 4’s they would, but most don’t like to think of Rupert as a half as that terrifies most of them as the child is always weaker than the parent.