If he likes Glenrothes, I would attempt to hunt down a bottle of
Glenrothes 1985. It's a Sherry Cask bottling. Very, very smooth. Pure butterscotch. I'm not a huge fan of what Glenrothes has been putting out recently, but this is one they did right. Should be about $120-$150ish
If you are unable to track down a bottle (and it'll be a tough one to find I imagine) you'll probably have to find a higher end Scotch/Wine shop, failing that, I personally, would contact the distillery or their rep to see where shipments of said bottles were sent to, and then sort out a way to get a bottle in your hands. But, that's just me, and I'm willing to do an awful lot for a good bottle of Scotch
If you're looking for different brands with a similar theme of vanilla/toffee: Balvenie is always a sure bet. And for the amount you're willing to spend you can deffinately afford an older whisky which will make your friends' eye's pop when he opens it (Although I personally am a huge believer in 'older isn't always better')
Sorry for the lengthy reply. I'm sure others will have great input. Good Luck