Please consider that experience and memory is subjective. While you may have not intended to force your son to do anything, he may have experienced that differently.
In my case, my parents never forced me to be a boy. Yet, they expected me to be one, they said a lot of things in that regard, they expressed their opinion every time I deviated from that, and they treated me assuming I was a boy. And they are awesome people I've always admired, so their opinion and a huge weight for me, bigger than they realised and probably more than it should.
So while they never forced me to do or not do anything, they did influence me a lot, and I ended up not doing so much of what I wanted, to do what they said they wanted.
They didn't force me... Yet I was forced. And that's why I had to go far far away from them to be able to transition.
Maybe you didn't force your son to dress girly, but by not expressly opening the possibility of him dressing more masculine, and him being only a kid whose view of the world depends on yours... he may have felt forced. Those feelings are as valid as yours. If you can validate those feelings, you can begin to heal a wound he has on his relationship with you, and you can begin to support him from there.
As to if it's real or a phase... If it doesn't go away it's not a phase. A therapist with experience in the subject (not in "curing" trans or gay people) can help him get his thoughts and emotions in order so he can find out what it is. But he's the only one who can write it off as a phase.
Thank you for seeking to know more. That itself is the biggest step to support him.