What kind of play? At 7 my parents wouldn’t have played with us – walks, bike rides, days out, board games, yes, but not pottering about at home stuff. They’d do DIY/lunch/the garden/whatever and we were expected to amuse ourselves: my mum’s mantra was “only boring people get bored”. And I couldn’t play with my sister because we didn’t get along, so I was on my own mostly. Separately she and I quickly learned how to entertain ourselves: crafts, digging holes in the garden, being a spy, colouring, making things, reading, helping (hindering) our parents in their tasks. We also moaned at being taken for walks but were made to go anyway and eventually that too “took”.
Perhaps I’m horrible but I’ve followed the same path with my DC: they can play, I’m in the vicinity but they do their thing and I do mine, unless it’s Monopoly or cards or something. Everyone has to go for a walk unless the wind is blowing you horizontal. They’re extremely self sufficient and can disappear for hours to occupy themselves. But I accept this may be a personality trait and I just got lucky/they inherited the ability from me.
Can you enrol in something together: swimming? Or break the day into chunks: TV and getting ready for the day, breakfast, he chooses the morning activity (staying at home boring you senseless), then it’s lunch, then you choose (a walk, or him occupying himself while you do what YOU want), then an hour together before dinner doing a specific thing like a game, then dinner, then whatever… basically the same way you’d get through the mind numbing tedium of looking after a baby, just approach it a bit at a time rather than staring down the barrel of a whole day.