I suspect it's the cocodamol, I take it and never have any problem, but a friend of mine finds that any time he takes any codeine preparation, he gets very constipated - and your OH is taking the maximum daily dose.
Rather than taking laxatives, can he try another painkiller (Neurofen or similar)? You can buy cocodamol over the counter so it's not a prescription medicine. Taking laxatives in volume might have too much of the desired effect!
Is he eating lots of fruit and leafy veg? Might be worth giving that a go, if he want to stay on the cocodamol, to counteract its slowing-up effects.