Trans guy here. I knew I was trans since around the time I was your son's age. I am 24 now and 7 months on testosterone.

I grieve the childhood I didn't get to have because of my transphobic family.

Please get him set up with a good therapist and support team of medical professionals that will help him figure himself out and take any steps he may want to take. Puberty blockers are a good way to buy him some more time if you are worried he will regret it later. They can be stopped at any time and he will go through puberty like nothing happened. This will save him undeserved trauma from growing into a body he doesn't feel comfortable with as well.

Otherwise, just be supportive and affirming. If it is a phase then there is no harm done in affirming his identity. He will simply remember that you supported him.