The first thing I noticed is that his wind-up delivery is really a combination of a normal wind-up and stretch deliveries. He starts off facing 3rd base but he still takes a step backward. I don’t think that’s illegal but I suppose it’s possible some umpires might not be use to seeing that and saying it is.
If you let him continue to use that delivery, I would recommend that he not open up toward home plate when he takes that step back. Right now, he opens up a bit and then closes back up when he lifts his knee. This creates a lot of unnecessary movement.
I don’t really see a problem with his arm slot. I certainly don’t see a low elbow. Instead I see a very over-top-top arm slot that is due to him tilting his head and leaning to the glove side. This is a posture issue that I’d recommend fixing by having him keep his head upright through release.
He does need more tempo and to build more momentum so that he gets out over the front foot more. I like to see the front foot, front knee, and chin alligned vertically at release with the release point out in front of the front foot.
Now having said all that, keep in mind that your son is very young and may lack the functional strength to do all of these things well. That’s ok. Just keep working at it and it will come in time.
I really like what he does with his glove. He puts it out front, turns it over as the shoulders rotate, and brings his chest to the glove. Very nice.