I so wish TT was an exact science. I truely don't think there is a way to get it exactly. Some people say regular sittings, others say watch for their signs.
When I was frustrated with DD and her progress it used to drive me nuts when people said 'she'll just click one day'...yeah right thought I, but I stopped pushing and forgot about it. One day at CC they put her in undies....within 24hours she had it.
My DS is the opposite to DD but similar to yours. He is 100% TT (figured out all on his own while I wasn't even watching)...until I try and put undies or shorts on him....then he has no idea. Not so great for out of home social settings lol.
I'm not going to worry for another few weeks. I honestly think my DS just isn't ready yet. I'm lucky enough though that nudie run weather is pretty much all year here, so he wears no pants at home. I'm just waiting for it to click.