@esther1977 He might enjoy short-circuit training. Higher reps shorter breaks not to failure.
Join excercises together without rest between like
- bicep cable curl and overhead cable tricep
- Machine chest fly and reverse chest fly
These can be done quickly and he might find more engaging
He will enjoy variety, and being challenged to a certain extent but he must be made to feel like he succeeded with each challenge.
Encourage a variety of activities not just weights at the gym. Some ideas, team sports football basketball martial arts, solo sports running swimming riding.
He might though want to be like his big brother so if you don't do these it will be harder to encourage him.
First form consistency and a habit, later grow the length and difficulty of each session.
Good on you for taking the time to think about how you can help your little brother. Wish I had a big brother like that.