CAVEAT: If he has vision problems, don't waste money on a maximum-resolution screen. The higher the resolution (pixels/inch), the smaller the fonts appear.
Of course, Mac even has a "Increase/Decrease font size" tool you can drag up into the Safari toolbar. Also, you can control font size with "Pinch in/out" gestures on the multi-touch trackpad, too... My eyes are failing, so I use those features all the time.
FWIW, I haven't used a mouse in years, The Mac trackpads are so good that I even use mine for drawing complex graphics -- including creating super-detailed maps.
The 17" is extra-nice for browsing/FReeping. I keep a pair of windows open side-by-side -- and with FR's "slave window" option, I have the forum in the left window and the current thread in the right Window.
Of course, I have nine "desktops" running simultaneously in Spaces. Right now, I'm toggling between FR. designing a presentation in Canvas, and monitoring a desktop with several weather radar screens running. (NE TX is under Tornado Watch -- and has 90 - 125 MPH straight-line winds predicted for this evening!)
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If it is what he has his heart/head set on, I don't see how you could go wrong with a 17" MacBook Pro... (For one thing, he'll never "outgrow" it... '-)
But do waste money on a large external monitor. I have an HP LP3065 (same size as Apple's 30-incher, but better quality at about two thirds the price). I leave the menu bar and the dock on the laptop's screen and position the external monitor logically to the right of the laptop display. That arrangement allows me to use the external monitor's full 1600-pixel height for application windows. E.g., I am now running Firefox 4 with only the Title Bar and the Add-on Bar showing. I have the location bar dragged into the Add-on Bar at the bottom of the screen. The whole rest of the window is devoted to the thread. And, with the 2560-pixel width of the HP monitor, plus the 1440x900 15-inch laptop screen, there is plenty of room to have additional browser windows and other apps open.