This Youtube video shows how to make one using poly pellets from the craft store:
http://www.youtube.com/watch?v=zp8MpA12n04
Also found this tutorial that uses poly pellets:
http://thesquishypickles.blogspot.co...-tutorial.html
I believe the actual weight you use is very important. Check with the OT to get a specific weight for the child. The tutorial says the standard is 10% of body weight plus 1 lb, but weights can vary also for the individual child. Or, perhaps weigh the blanket he uses at school and replicate that weight and placement of the weights in the blanket.
Here's another tutorial with good photos (also uses the poly pellets):
http://dandelionmama.com/2010/01/16/...ghted-blanket/
Edit: Oh, and I found plastic pellets here on Amazon:
http://www.amazon.com/Darice-Bean-Fi...dp/B001O5SQCO/
However, I don't know if they're the good polypropylene ones that don't melt. Also, one of the reviewers said that Walmart had the same ones for half the price.
Also found these on Amazon. Reviewer says she uses them to make weighted blankets, but that they are a lot less expensive purchased locally:
http://www.amazon.com/Fairfield-Poly...dp/B000E3E6GM/