Fits the bill for functional constipation - there is a backup of hard poo, but newer soft poo goes past this and is passed. The old dried backed-up poo can cause bloating and stretches the rectum, and the hard mass can put pressure on the bladder.
It’s not always well understood or treated in the UK - many other countries X-ray for it.
If it is functional constipation he will need a disimpaction regime - this is a big dose of something like movicol, not a small daily maintenance dose as can be incorrectly prescribed.
Hope you get a good practitioner. In the meantime, fill him up with tonnes of fluid, fats and fruity fiber - coconut milk, peanut butter, olive oil, butter, prunes, pears, kiwis, baked beans, ice lollies, smoothies
Warm drinks are AMAZING - hot chocolate, hot lemon honey, warm juice - they will really move things.
Go easy on milk and bread and other wheat products.