Don't take this the wrong way, but I would never get a 7 year old a 20 gauge semi. It teaches bad habits. There is a lot to be said about waiting for the right shot that is instilled with a single shot. Plus I'm not sure you'll find even a youth 20 gauge semi that will fit a 7 year old. If so, it certainly won't be balanced.
I'd wait until he is around 10 or 11. Until then, stick with that 410 a little longer or transition to a 20 in a single shot.
The semi can be single loaded, growing up I had a couple friends that had semi auto as a first gun and they single loaded.
The old 1100's were pretty soft shooting and could be had in youth models. I have a LW 1100 in 28 gauge and it's a pleasure to shoot, much easier wing shooting than my 1100 or 870 410's.