I'm new on this app today. I'm the owner of a guitar store, but I'm really a sax player. (It's Yanagisawa, BTW.)  

So here's the deal as I see it...

If he's going into music as a career, it's time to get him a better horn. Keep the AS600, but get a better horn. Think used, as well as new. In many cases a used horn, rebuilt/refreshed by a professional shop will be cheaper and better than a new horn in the intermediate level.

Before we talk brand and cost, you need to think about mouthpieces. Get the best mouthpiece you can afford for the style of music you intend to focus on, and if that is too expensive, buy it anyway and keep the AS600 for now. A great saxophone needs a great mouthpiece.

The Yanagisawa Brand is excellent, and more affordible than Selmer's Pro series. (Mark VI at 15,000!!) But they are different in sound and response from Selmer. (All brands are different) That could be good or bad, but he needs to try the horns he's thinking about to determine if what he hears, he likes. All horn companies target different audiences..../ styles.

In the Selmer line there are the incredibibly high priced pro horns, but there are also some intermediate horns in the used market that are affordable. The AS42, the AS210, were pro level horns made in the past, in America. They are available used, and a big step up from the AS600. (800 - 2300)  New at same price range the AS411, AS711.

The Yamaha Saxes are another brand to consider. I recently worked on one of these and I found it really sweet. They have an intermediate series retailing around 3800-4000. Used should be more affordable.

I am playing an intermediate Jupiter Saxophone currently. On their pro line they are affordable and really beautiful. Similar price ranges, but used they can be a steal. Not so many people seek them out. 

Then there are the Cannonball Saxes. which are in a field all of their own. If you play in a dance band or rock band.... 

Try them if you can. If you have a pro sax shop nearby, call them. They may have some used, rebuilt saxes or know of them. Also find out what they think about different brands or different models in a brand. 

Good luck