I wouldn't worry about it too much. For a while, T can be disguised as a cold. And for some people it takes longer to get really visible changes.
If you are planning to keep it a secret from him, I would make sure you have a place to hide your T and any supplies you'll need. I would also do a little inventory of your nearby relatives to see if anyone would potentially have room for you in their home if your dad does find out and react poorly.
And if you haven't been working on it, I would also continue to look into getting your monthly benefits fixed so that they're going to you instead of him.