Sparkly,
This is a clear cut case, I'm afraid, of the NHS treating those who shout loudest.
Yes they prefer not to do joint replacements on younger people, because they do wear out and need replacing again, which is not only expensive, but technically more difficult. But my surgeon took the view that if my mobility was SOO severly compromised then it was probably better to do the replacement now, and worry about the further replacement when needed. Probably the technology will have improved by then anyway, and replacing the replacements will be less problematic than it seems today.
Your dh needs to go back to his consultant, and point out that he can't go on like this. His quality of life is VERY badly affected, it's affecting his ability to partake in family activities, can't drive, etc etc.
He needs to persuade them to do something, not just let them send him away with 'nothing more can be done', which suits their budgetry constraints.
Is he doing everything he can to help himself? If he's overweight, he needs to lose it, which will help his knees, and make the surgeon more sympathetic to his case.
Also try taking cod liver oil and/or glucosamine tablets, which may ease things a bit.