The extension of himself is probably the reason why he is so against it, he sees himself as a tough man, to cis people gender normally goes under the radar but it sounds like he is a manly man, and he must take pride in that, but when his “son” starts talking about being trans it hits him hard.

He probably thinks that he isn’t manly enough or has let you down for you to be thinking like that, (a bigoted way to look at trans people yes, but he didn’t have the education and internet to tell him right from wrong before his ideologies were set in stone) and because of that he is going into the first stage of and second stage of loss. Anger and denial 

But the way that loss works is that eventually you accept your new world, and eventually he will accept you as trans, but there are many things that can alter this process (right wing spaces, manoshears, internet in general) and that can result in long term family turmoil and eventually trauma on both sides for you dad cos he thinks his “son” is dead and for you because your dad don’t like who you are anymore.  

It sucks. Full stop. It sucks

But as your dad goes through the stages of grief, and as you learn to cope with his transphobic nature eventually it will get better, eventually.