The one he's using above in the images looks like the one from National Brand [1]. I recognize it because I use it to this day for all my work. Ever since engineering classes at RPI, it just helps organize my writing/sketching and for subsequent filing/reviewing. Highly recommend :)

1. National Brand Computation Pad, Plain and 5 X 5 Quad On Back, Green Paper, 8.5 x 11 Inches, 200 Sheets (42389) https://www.amazon.com/dp/B0017TMB64/ref=cm_sw_r_awd_pbRYvbT...