Toddler will never be potty trained

I swear this kid is killing me. He's almost 3. We've been loosely potty training for a while, just to get him comfortable. So maybe 1-2 times a day he would sit there for a few minutes with no expectations. Every now and then he would go. Yay! Except this week we decided to reeeeeally try. I've done everything I can think of to encourage him to go, to no avail. He's chock full of water, I let him watch videos on my phone, but nothing. He just sits there. He sits well but doesn't go. I don't understand how he can sit there for 20 minutes and nothing comes out! We were going every 30-ish minutes for around 5 minutes at a time and he only went once.

It's frustrating and I don't know what to do. He says he wants to wear big boy underwear (my mom got him paw patrol training underwear for christmas) but still doesn't actually go.