Yep, that will do. I use light weight non-iron-on interfacing that handles like paper - the sort that gets sewn into clothing. The foundation stays in the block, the backing is tied onto the joined blocks, and there is usually no need for batting (although you can use it if you want).
If you want batting I found it easier to start youngsters off using fleecy knit fabric, although modern battings are more able to withstand "handling" by kids than the stuff we had back then lol.
Manx quilting is very old - the origins of log cabin. There are instructions here -
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- but do not try to learn this method together lol. Make sure you are familiar with it before teaching him. It's easy once you make the first block. And while the traditional method calls for strips to be torn, you could pre-cut the strips for him. The big advantage of Manx quilting is that the pieces are "designed" to fit in small hands. Making these quilts was the job of the children of the household. It was how the young (mostly) girls learned to sew.
WARNING - the first block will be SLOW, but it gets quicker after that. If patience (or lack thereof) is a factor, draw foundations with only three rounds instead of four, and make the logs bigger = quicker results.
It he sticks it out for six blocks you can make a table/place mat like a mini-quilt. All the construction techniques with only a few blocks. BTW, traditionally Manx quilts had turned edges (backing wrapped/folded over to the front and stitched down) but there is no reason you couldn't use other binding methods if preferred.
Good luck. I used to teach cubs (juniour Scouts?) to sew and I loved it. Probably a third of them went on with their lessons and made simple items at home back then. They seemed more likely to finish if the item was destined for a specific use or a gift for someone. The others lost interest after the first block or two - the lure of fishing and football rofl!