The way I do my illustration practice, good ol' HB pencil is my main non-digital sketching tool. For me, pencil sketches are done for studies, planning and underdrawings.

2B is really good for life drawing, especially if you're drawing on larger paper because it goes a bit darker and wears down to a thicker line more quickly. It makes a more visible line and it's still hard enough to work with cleanly... not too smudgy. It's really great for life drawing in a larger sketchbook, say 8.5x11 or up to 11x14.

If he's drawing really big, Charcoal or Black chalk are great options for life drawing. A pencil line will be too small to see well on a big page... maybe handy for getting into small details on that big page, in which case a very soft pencil would be able to go dark enough to show up with all that black charcoal.

As far as lead hardness goes: the softer you go the blacker (B) it gets, and the smudgier and messier it gets, and the harder (H) you go the lighter and more difficult it is to see. HB is the middle of the range (It's Hard and Black in balance) and 2B (the next or 2nd level of Black) also gives you a lot of tonal range to work with.

Sometimes I use blue-pencil, which is a comic book artist's or illustrator's tool for making drawings that are meant to be inked over. It's pretty cool to use because it changes my approach to drawing. It forces an artist to draw lightly. It's often tempting to go too dark too soon, but the blue pencil can't go dark, so it teaches us to change our approach. And even though it's very light, it's also easy to see because the blue hue adds a visual intensity to the line...stimulates the colour receptors in our eye.