Is It Safe For My 9 Year Old To Lift Weights To Build His Upper Body For Baseball?

Yes, provided the weights are light and geared for repetitions rather than power lifting . He is still growing and his bones can be damaged along with surrounding tissues. Using his baseball bat as the weight would be ideal. Get him to stand upright with legs apart and hold the bat at a 45 degree angle  from his body. Then changing arms and trying to increase the time he can hold the position comfortably. Another useful exercise is to stand with legs together and both arms straight ahead of him. As if pushing against an invisible wall. Then as he stretches his arms forward he should try opening and closing his fists tightly . After about 2 minutes he will feel the muscles in his arms tighten up and the heat spreading to his neck arms and wrists. Try to increase how many openings and closing of the hands keeping arms extended.  This was a judo exercise I used to improve my grip on my opponent and my arm strength to move him around the mat. If he does this right he will soon see and feel the difference. Its surprisingly effective.   So this way the only weights he will be using are his own arms. :)

Not at such a young age.
However this may sound stupid but will be safe for him to do and will make a difference over time to his strength.Try getting him to lift 500ml bottles filled with water in classic right angled rep style.
Then over time get him to use both arms with a 3 litre bottle filled with water.
Press ups will also be fine but start slowly on his knees not toes.

If his doctor says he is in good enough health I don't see why he can't. My brother and 2 sisters took karate, taekwondo, and lifted weights at an even younger age than your son. As long as he paces himself and does it properly with a knowledgeable trainer he should be fine.

I think it is a Great idea to have your son lift. I am 12 years old and I lift, and I have a cousin that is nine and he lifts. If your son wants to gain muscle and strength then let him do it. He should do bench press and bicep curls-he should start with a quarter of his own body weight for benching, and start with one tenth of his weight for curls. Every week increase the weight by 2-5 lbs. He should lift about 3-5 times a week. If he can't handle the weight getting heavier this fast then change the weight every two weeks. Over time he will defiantly build more muscle and increase his strength.