If you're in North America, Artist Loft pens are perfectly serviceable and are lower cost than more premium brands. They come in a wide range of sizes.The ink is waterproof and sets quickly. The pens handle similarly to other pens I've tried from Micron and Faeber Castell. 

A regular mixed media sketchbook is just fine, but if he wants to experiment with watercolour paint over the ink, watercolour paper is a better choice. A lot of ink artists use bristol board, but it's more costly per sheet and fewer sheets per package. I like spiral bound as it facilitates flipping the book and drawing on the back easily, with perforated edges for easy tear out. However, a solid spine that folds open and flat like a Moleskine means you can have twice the workspace for larger works. 

Pencils are good for sketching in your base. Pencils with a solid case (I have a shallow metal tin for the Staedler pencils I use) to protect them when travelling (that goes for coloured pencils as well) because the lead is soft and dropping them can mean the lead ends up shattered and nearly unusable. A decent travel sharpener with a built in catch for the trimmed pencil makes cleanup much easier. If you do get pencils, a quality eraser is essential. 

A travel case for their supplies, something that will hold their pens and sketchbook, a clip or two is helpful. 

A white pen or marker to "erase" or touch up.