Hi there,
Depends on what your current laptop is whether it's more capable. I'd suggest going for at least the base MacBook Pro, which is $1499. If you don't mind spending a bit more and going to $1799, you get quite a bit more machine. I'm sure he'd be fine with the base, but the extra $300 now might get you some extra life out of it as it's a bit more future-proofed with the higher specs.
Depending on how much he wants to do, and if your budget really is the $1500, the higher end MacBook (non-Pro) should suffice as well. It's $1,599 and has more on-board storage which could come in handy for storing images. It might take a bit long to do some tasks than the Pro, but is still capable at handling digital photos.
If you have an Apple Store nearby, stop by one and try them. Go to a site like
Steve's Digicams and read the reviews. They have full sized images on there that you can try opening, import to iPhoto, etc., in the store to help get a bit of a feel for the machine.