Prune juice works a treat but if he doesn’t like/you want something more gentle try dried apricots.
Soup wise avoid root veg go for pulses, beans, tomatoes, greens, peppers, onions & related, beetroot, pumpkin/squash
Also increase fluid uptake BUT no caffeine or fizz.
Caffeine is a diuretic and the fluids will move through his system too quickly to properly enter cells and gut, it can relieve quickly but then things stop up again
Fizz when constipated often leads to trapped wind which is awful!
Bananas are not good for constipation, they don’t actually have much fibre, too dry and hard to digest plus they can lead to gas production leading to trapped wind again
Wholemeal/bran type things can dry you out, I’ve found they’re best employed just as the constipation starts to be relieved
Glad he’s feeling better but I’m still posting as may help in future or someone else