Trying to put together the timeline, lets say 6 years out of college, so you're maybe in your early thirties. Let's say your Dad had you in his late 20s, early 30s. So he was in his 40s when you and him founded the business, and you were a teen.
Maybe he doesn't think you are ready yet. Maybe he remembered all the mistakes he made in his thirties, and how in his mid forties everything came together when you helped him move online and save his business. But for a business man, all you did was tech stuff. He has seen you follow the start up path out of College and spend 4 years chasing the start-up-lottery dream, and doesn't want to give you $1.5M for you to continue chasing (in his mind) smoke and mirrors.
Maybe while you spent 4 years in college, and he was running the business, with programming employees, he found out that you created a serious hairy mudball that no one but you would ever understand. (I remember some of the first sites and software I made in the early 90s when I first learned web programming and kept building on them because they worked and were in production which now make me shudder to think of).
It might be that he thinks you still have fire to burn and things to prove, and are not ready to take over a company that is running well - that you might makes changes just for the sake of change.
Maybe he fears with more control you would kill the goose that lays the golden eggs.
And he could be very wrong. Made worse by the idea that maybe his assumptions are all wrong, and he is not doing this out of malice (as some commenters are quick to point out) but he genuinely believes he is helping you.
Who knows? We have your side of the story, and though I believe your side is true, it is just half of the story.
/Devil's Advocate
I wish you the best of luck. And I do think your father has some explaining to do.
(My view is formed by my being in my late 40s, having a 6 year old son who I love, and trying to think why my son in 25 years could think that I am a great person, but also think I am screwing him over).
Edit: Spelling