Please please please have him really stop and think about this. My husband and I have spent our whole lives riding bikes (motorcycles) and there is 1) a lot of safety to learn if you've never ridden before, 2) it's a lot of money, not just buying it but fixing, upgrading, etc 3) lots of time, if he plans to ride it for fun and unless your kids are older and can stay home by themselves, you will be home with the kids while he's out riding.
Here's an example. The husband of one of my friends decided he wanted a motorcycle. He just went out and bought one, never ridden before, doesn't know the first thing about them. He had to call my husband to come pick it up and ride it home for him! Three years later, he never rides it, it's not even worth what he bought for it, it sits in his garage and takes up space.
Another example. An ex-boyfriend of mine went out and bought a dirt bike because that's what me and my family ride. We had to teach him how to ride and then after only a month he decides he's pro, goes out jumping it and breaks his collarbone.
I'm not saying your husband will do any of the above but he needs to think long and hard about everything that it entails. We were talking about selling one of our bikes, my husband's GXR1200 and he said he could get $5000 for it easy, so $2500 for a 600 may be farfetched. Don’t forget the extra licensing, taxes and insurance that will need to be obtained. Some states (not sure about CA but in CT) you can't even get your bike license until you've passed the DMV test. Does he have a friend or family member that rides that can show him what to look for in a bike? What to check for in a used one? How to ride?
I'm not against riding, everyone in my family (immediate and in-laws) does it. However, we want people to be safe and know what they're getting themselves into. Good luck!