for my older son they didn't have tackle until he was in 4th grade so he started then, for my younger son they had expanded and then had it down to 1st grade and that is where it still is and he started in 1st grade.
I see no reason at all to play flag football, no reason not to start them playing tackle as early as you can. I spent a lot of year coaching youth football and and never saw a serious injury to young players. It wasn't until they hit the jr high level where we started to see some more serious injuries and even those were pretty rare. With the small kids especially they just aren't fast enough, strong enough, or athletic enough, to really hit each other hard and cause injuries. So the people that try to tell you that they shouldn't start until they are older don't really know what they are talking about, when they are older they are faster, stronger, more athletic, and hit much harder, that is when serious injuries start to happen not at the young ages.
Let him play tackle, the odds are so slim he'd suffer a big injury that it is not a concern.
And my kids played for American Fork, I coached, so I'm pretty familiar with the league and how things are done. Get him started it will be fun for him and you.
If you want to BM about it me feel free.