Seriously, it's just perseverance. It took DS1 (who's now five and a half) six days to "get it" at the age of 2 years and 7 months. My DS2 I TTd in five days - he's a month older than your DS. I seriously just recommend perseverance. I did a post in this forum (somewhere) about TT DS2... follow that along, I recorded day by day what he was up to.

Keep it up. One day it will just "click". I swear I was going to give up after Day 2, but I kept going, and he "got it" on Day 4...

HTH.