Recognizing/hitting curve balls

Look for the ball coming out of the top of the hand.

I have learned through many years of hitting and now coaching in my first year at the Freshman level that about 99% of the pitchers slow their whole motion down when they are throwing a curveball so I would tell your son to look for a real slow motion from the start of the pitcher's motion to the plate.

Agree with BigHit. Also tell him to watch the seams closely. The seams on a fastball spin backwards and on a curveball it rolls forward (12/6).
As far as teaching him to recognize when it's a strike, I have a drill I used when we first started seeing curves. I have the batter stand in the box with his glove instead of his bat. I used the curveball machine to get precise curves and made them catch the ball and hold it to see where it crossed the plate. It seemed to help them visualize the path the ball would take. I also made them stand up in the box for a while, while they were getting used to the curve. This allowed them to reduce the amount of break on the pitch. It made them vulnerable to the fastball though. I instructed them to look fastball and adjust to the curve.

Learn to have a plan at the plate and look for your pitch with less than 2 strikes (fastball belt high down the middle). Recognize curve ball and don't swing with less than 2. Anticipate fastball away with 2 strikes and adjust to curveball. Works for me and mine!!

I would add that he needs to practice hitting curves. Tell them they are coming and then pitch them. After many reps you could mix them in without telling him.

Also, he should be learning to pick it up while others are batting. In the dugout he can still look for signs of a curve.

My son is 14U. Teaching him to hit the curveball has been a very simple process over the last three years. I just kept telling him that he could hit them.

Every once in a while, even though he thought that it was beneath him, I convinced him to go hit in the little kids speed cage so he wouldn't just 'groove' his swing in the faster cages.

Other than that, I didn't do much. For him its been a matter of believing that hitting curves is no problem. Last year, wo to the pitcher who threw 2 strike curves on the outside corner. The hits were either grass burners up the first base line unless the pitch hung, then a double in the gap. I can't think of but only a very few weak bouncers to the SS. He now thinks that he is a good two strike hitter as well.

He just does it. He 'thinks' it easy.

He is doing right the most important thing, ready for fastball! So, because curves and changes are slow pitches, will be easy for him to react to them. Tell him to keeping his approch to fastball and hold the swing to the last moment, that way he can reconize the breaking of the pitch.