RC for dad and 7 year old...good move dad he will love it
The Sig LT 40 with your motor will be perfect. One really good reason for the LT 40 is it will fit a 7 year old's developmental level if set to fly at a low throttle setting...more reaction time. I built one for my son but he was 16 and was more into cars and girls soooo its MY favorite plane now. The instructions and great fitting parts of any Sig kit will enhance the fun you and your kid are going to have.

Some dad to dad heads up here. You may experience some issues with a seven year old's ability to stick with a long build late in the flying season. depends on if he knows about this yet??? A trainer kit takes an average of 40 hours to build maybe a little longer with a kid helping (He will never forget this though so do it by all means) just don't overwhelm him with long build sessions. I think you already have this down because of you said you want him to do most of the work. Don't forget to take some pics! Especially when one of you glues your fingers to the plane[&:]

You could download some pics of covering schemes that you could live with and let him choose one.

if you haven't told him yet you might take him to the local flying club to watch to get him fired up. if you have told him then you HAVE to take him to the flying field(just kidding).