I think Shiraha gave all his powers to save Sousuke, but if he did that, he would have died, so his father probably gave half his powers up in order to prevent Siraha from dying. That part of his powers probably prevented him from being able to transform into human form anymore because they are meant to look like deer when they have their powers to mate with a doe and make a successor. I think it's just easy to sometimes mix up Siraha and his father since they do look similar when he looks proper, but his dad has always been in the form of a deer since he gave up half his powers to save his only son since his wife died in order to give birth to him.
Thank you!!! I didn't realise I missed so much of the plot.(づ￣ ³￣)づ
You didn't. That was never actually stated
I just don't get the part about Shiraha's father. Why is he a full on deer in the last chapter? Did he transform for the sole purpose of being a dick and not being able to answer? I'm sorry maybe I just didn't pay attention.