Alright. Here's how it's gonna be. Your kid may or may not want to start playing music. If he wants to, start him off at age 8. With piano. Yes, you might want to have him play bass, but it's just ideal to start with piano; it's a great entry-level instrument. From it, he can learn theory as well as start making sounds. Get him a teacher too.
Then, after a few years of piano, ask him if he wants to continue. If he says yes, then great. If he says no, then don't force him. EVER. He'll just hate you for it. Now, back to the yes. If he does say yes, ask him if he wants to learn bass. If he says no, then don't force him or else he'll hate bass, that's how it is. Just let him continue on piano or whatever else he wants to play.
If he does want to continue music and he does want to play bass, then get your wallet out and go to the store to get him an entry-level bass. No need for a Sadowsky or whatever. Just a good beginner entry-level one.
Have fun!