3 point FG good, or NOT?

[thumpferee]

Situation. 3 man. HS VBoys. 4th Qtr. Close game.

I am L in transition. A1 gets the ball deep in the corner. I get wide. A1 fakes a drive and pulls up for a three. I sneak a peak at his feet and the shot is clearly a 3. As A1 is in the air on the shot, I focus on activity underneath. The ball goes in. I look up to verify my p signals the 3, he doesn't. I whistle and stop play to inform my p what I saw. I tell him his feet were definately behind the 3 point arc. He tells me it is his call as the T and since he didn't signal it as a 3, it should count as a 2. He did not change it to a 3.

How should this have been handled?

Thanks!