This may look silly and I didn’t learn uni this way but seeing my son struggle made me build this to help build his confidence learning uni. Any suggestions?
Yes, let him out of the box.
He’s trapped! He has something to hold onto, but he can’t go anywhere. He needs to learn how to get the pedal past the bottom of the stroke, and that’s not going to happen with such a limited space.
Reconfigure it into a strong railing and it will be much more useful. 
How about making something like a walker with four wheels? You could just add wheels to your box.
he needs to be able to move.
Here is someone else’s solution:
How long has he been struggling? Be patient Allow him plenty of time to learn it himself. I taught my boy when he was 7. I made sure he was well padded up. No devices needed, they just end up too dependent on them.
I have a long van in my driveway. I have it parked next to the fence. I had him practice rocking back and forth for quite some time. I made sure he got a good feel for the rocking motion. Then I had him travel back and forth with with the assistance of 2 hands. When he was comfortable to let go, I would tell him to let go. Then came more rocking back and forth. Everyday I would have him practice rocking back and forth, it is so crucial. Then he would move over to the other side of the van where he has only arm for support. By now he was comfortable to go back and forth, but wobbly of course. When he was comfortable I had him launch off from the van. Fortunately, my boy likes to fall. I bought him plenty of padding, even though he would sometimes get scratched up, he would always get back up and do it again.
I think you just have to take it very gradual, you just have to let them gain the confidence in falling and that it wont hurt so bad as they might think, given the fact that you provided plenty of padding for them…
He’s young and should learn quick with no aids except a wall or railing and a lot of encouragement. If something is needed, I still like the shopping cart. Not so cool looking, but he won’t need it long, and it will be extra incentive to learn 
Knowing how to cut and glue pvc pipe is a useful skill, might serve him well!
When I was first trying to let go of the wall and ride into space, I discovered that having something solid about one crank turn away helped a lot, something I could put my hand on to steady myself as I passed. There was a chronic wobble the first time the pedals went through 12 and 6 and I had a hard time getting past that. Your fixture might help a lot there.
Cool kid! How’s he progressing? I imagine he’ll get pretty comfortable sitting on the uni in that contraption and be ready to ride soon.
My kid was also 8 when he learned to ride. I set up two rows of chairs with the backs of the chairs facing each other to make an aisle between the two rows. (Does that make sense? I shouldn’t internet before coffee.) He’d ride down the aisle and have a chair to grab on either side. Each day I’d move the chairs a little bit apart so he’d have to ride farther in order to get to the next one. Within a week we got rid of one side of chairs and in another week he was riding unassisted. We didn’t have a fence or deck rail or anything like that to use but the chair thing ended up working out perfectly. Just an idea for you when your guy is ready for the next step.