Is 120 a good IQ score for a 9 year old boy?
120 is a good IQ score for any age. It means you are above average - the average is supposed to be 100 for any age group.
Any IQ above 100 is good, it means that you are above average. 119 is substantially above average. Your age is already taken into account when the IQ score was calculated, that 119 is how you rank compared to other 7 year olds, and doesn't say how smart you are compared adults or kids of other ages.
Age doesn't matter when determining whether and IQ score is good or not. 160 is extremely good for any age. This is the estimated IQ of Albert Einstein. If this score is from an online test, I would advise you to not believe it. These tests are not accurate. You need to take a real test if you want to determine your actual IQ.
IQ tests are supposed to be based on the age of the person being tested. The median score being 100 with 15 as the standard deviation. The 9-yo boy in question would be above average (assuming he took an age-appropriate test), but by no means genius. A high score or a genius score is around three standard deviation from the mean, or 145.