Honestly, I would just get him a gift certificate to a running store or better yet a homemade "you find what equipment you want and I'll buy it" certificate. This type of gear is so personal that finding the perfect thing for someone else is virtually impossible. For instance, some people love handheld water bottles.
I would rather die of thirst than carry something in my hand during a run. I tend to carry a lot of fuel and other stuff when I run so a running belt works well for me. Other people hate them. I need to have my phone accessible (I use it for my intervals and will flip between music and podcasts) so I wear shorts with pockets and, (no joke) when upgrading my phone the last time, measured these pockets so I knew what phones would fit in them. And just look at the recent discussion about hats to see how many different types, styles and sizes there are.
A marathon (let alone marathon training) is too long to deal with a piece of gear that doesn't quite work perfectly for you, IMHO. If you want to get him started on places to look, you can always do some window shopping in advance (or maybe some actual shopping for you
) and give him a list of links.