lock up the TV, have a pre-set time for viewing, give kids turns for picking the show, perhaps one hour a day or less, the rest of the time TV is OFF (unless dad wants to watch a game). Also put computer off limits unless they are actually doing school work and always monitor computer use. Grandson has to be dragged kicking and screaming away from the computer. Also limit time with video games, and dad keeps the controls.
martial arts is a good idea, also gymnastics, swimming, some of the sports where an individual tries to increase his best performance, rather than competitive team sports.
we had a rule that each kid had to do one active sport and one other activity each school year, i.e., gymnastics plus scouts, or volleyball plus band.
how active are you, your wife and the other kids, there is great dad-kid quality time throwing a ball around, riding bikes, walk around the block, cleaning the yard together etc.
would he be interested in gardening? grandkids take care of a garden plot, started when oldest was 3, they each have jobs to do in the garden each week, and surprise, surprise, actually eat the vegetables they raise.
also don’t make their room so plush they love to hang out there, no TV, etc., have the kids play outside after school and start homework later, they need the exercise, after a healthy snack. Ideally they should by outside until it starts to get dark.
the last issue of either RD or Prevention had parent-kids outdoor exercises to do together to get kids moving.
we stopped buying toys for them, and contribute to one outdoor item a year, the first year a climbing gym, then little tykes slide, last year a trampoline which they use a lot.
DD got that dance pad thing for exercise, the kids like it so much she had to order another one for them.