God created everything—and yes, you can tell your son that this means He also created the dinosaurs. They aren't mentioned in the Bible, however, since they were extinct by the time it was written. I also don't know why God let them become extinct (although I'm not sorry He did; I'd hate to have a hungry dinosaur peek over the fence at me!).
The Bible makes it clear that nothing happened on earth by accident; everything was created by God, and was in accordance with His perfect design. The very first verse of the Bible says, "In the beginning God created the heavens and the earth" (Genesis 1:1). The Bible also tells us that Christ was given the assignment of actually creating the universe, and "without him nothing was made that has been made" (John 1:3).
In time, your son will probably outgrow his interest in dinosaurs—but I hope he will never outgrow his interest in God and what He has done for us in Jesus Christ. Pray regularly for your son; God has entrusted him to you and given you the privilege of raising him.
Make it your goal, therefore, not just to help him grow physically and mentally, but spiritually, as well. Is Christ the center and foundation of your life, and over the years will your son see that Christ makes a difference in the way you live?