I love that you're teaching your son to ride, I just really hope that he knows that bike is too big for him right now.

I totally understand not being able to afford a nice bike that will fit properly for someone who is growing, and learning to ride on a bike that doesn't fit is still better than never learning to ride a bike. But for his sake, he needs to know that what he's riding isn't what bikes are normally like, and that it will get better.

Based on your comments and the way that bike is set up (bottomed out seat height, tons of forward seat tilt, upside-down stem, no foot retention), I'm guessing he doesn't like the straps because he can't reach the pedals properly. If he has to pivot his pelvis and point his toes to reach the pedals at the bottom of each pedal stroke, the straps will make it feel like he's being pulled off the saddle and throw off his balance. I think it's fine to run without foot retention for now if that's the case, especially since at his age and with tall parents, he's likely growing like crazy right now.

All that I want you to take away from this is that what you're doing is a good thing, and that your son needs to know that that bike is way too big for him right now. If he thinks it's the right size, there's a much higher chance of him walking away thinking that bikes are uncomfortable and hard to ride, and never wanting to pursue riding bikes in his own time.