He definitely needs a project!
The gardening idea sounds great.
Does he have any interest in crafts of any kind?
Up cycling a piece of charity shop furniture for his bedroom?
Learning to cook - are some good children's cook books where he could teach himself and cook lunch for you all.
Could he earn pocket money by doing household chores during the day so you have more time together after work?
Is there anything he could get involved with to help with your work in a similar way?
Has he checked local sports clubs, youth clubs, schools, churches etc to see if there is anything going on in the way of cheap summer activities?
Do you have any elderky neighbours it would be safe for him to visit or help? The same with a neighbour with much younger children who might be grateful of an older child go play with them while they have a rest/ get some ironing done etc.
Sewing name labels on his new uniform