Here is a hand-out I came up with for children pressing too lightly...if you have ideas you want to add, please comment. This is a combo of things I've seen on the Internet + a few of my own.
Too light pencil pressure:
1) Before starting to write, provide lots of proprioceptive input to the upper body. Crab walks, animal walks, downward/upward dog yoga poses, straightening elbows and pressing straight down onto desk (weight bearing into arms) 10 times, having him push his hands together, and/or chair pushups where he puts his hands on his chair and lifts up his bottom x 10, are all great ways to do this. The more input the better.
2) Consider trying a #1 lead pencil rather than a #2 pencil as it requires less pressure.
3) Practice fine motor activities in general, including cutting, lacing beads, theraputty or play dough activities, using tweezers/tongs, finger puppets, buttoning, zipping, etc.
4) Consider buying a weighted pencil, or making your own by supergluing or duct-taping some type of weight (perhaps using a nut of a bolt/nut combo, or some heavy washers) to the pencil, higher than his grip would be, of course
5) Try using tiny golf pencils, and/or if he doesn't like it, try using thicker/wider pencils.
6) Get some blank carbon copy paper to put under his writing so that he can send a copy of his writing to a loved one, and it requires a lot of pressure to show up!
7) Before writing, regularly use his pencil (both of you do this) to show a scale of "too light, just right, too dark" where you use shades of penciling to demonstrate this. Doing it regularly before writing will help remind him.
8) After he writes, go through and point to each letter. Is this a ghost letter or a just right letter? Let him count the letters that are just right, and that's how many minutes he gets for free choice time, or how many skittles he gets, or some other reward you want to use.