Welcome to the forum. As far as a replacement for your venerable '99 Stryker, that would naturally be another. Made in abundant numbers check the knife forums, eBay and you may find one. For modern replacements, seek out the Nitrous (assisted) Model 912. This has been discontinued as well, but they're around, and would be familiar to you being locking liner like your 910S. Now there's a new model just arriving for 2014 that's got a lot of attention. I don't know if it's hit the street yet, but this would be familiar to you, and it's got the advantage of ugraded blade steel and best of all (for many of us) is the added Axis lock, which is about the best thing going for Benchmade; it's one of the strongest locks on the market and proven.
I know that's a lot to think about but the point is, there are some great options available...and your Grandson sounds like he's going to be lucky at the end of this.
One thing I would recommend: before turning that 910S over to him, you might consider sending the knife up to Benchmade for what they call a 'LifeSharp' tuneup where they'll sharpen the blade and give the knife a thorough going over. Invest $5 for return shipping and pay to ship it up there, that's a great service!
Good luck, hope this helps.