Depending on what he has and doesn't have I can suggest some things for you.
A dependable PFD. (He probably already has this, but a good paddler-friendly PFD is absolutely necessary.) Flush Rod Holders
(If the kayak is not equipped with them already) Mounted Rod Holder
(Scotty is a really good brand, they have a lot of quality stuff.) Anchor Trolley
(I will note that a DIY trolley works just as good and is waaaay cheaper than these kits. These trolleys make anchoring a breeze and you can move the anchor point from one end of the kayak to the other. YakGear is a solid brand.)
An anchor. (Mushroom style, or DIY. There's a lot of styles. I normally just use a stick anchor pin or a small collapsible. You don't have to get a large anchor, the small ones work wonders.) Waterproof Cellphone/Wallet/Whatever Bags
(Mega recommend these if he wants to take pictures. Don't have to worry about dropping your phone into the abyss.)
A nice seat. (There's a lot of options for this and a lot of good DIY options. Pretty much whatever works. A good seat is very important and will become a top priority if you're on the water for a decent amount of time.) Paddle and Rod Leash
(The rod ones I linked aren't long enough to use the rod and secure it. They're good for trolling with a rod while in the flush mount. The paddle leash is perfect. I usually have my rods leashed while moving, and unleash them when I find the spot. I just put the rod between my legs when readjusting between casts.) Lanyards with Carabiners
(Not necessary, but I have fishing tools attached to these on my kayak's padeyes.)
Scupper Hole Plugs (If applicable, you can buy practice foam golf balls for use as universal scupper plugs so he doesn't get wet. They're tight enough for a seal, and not too hard to get out if you need to bail some water. You can also buy commercial ones, but a pack of eight practice golf balls is $5.) Paddle Clip
(Saves a lot of time.) Crate for Gear
(Obviously you can get a cheaper milk crate, but this was just an example. Easy to carry gear and retrieve it. Usually sits on the back.) Kayak Cart
(These can be a mega life-saver when you have to move the kayak a bit of a distance, or up and over hills...)
Hope this helped!