Usually when giving advice on here I spend much of my time trying to put people off claiming constructive dismissal because it can be very difficult to prove and as compensation is only based on actual financial loss, often not financially worth it either.
However, in your DH's case it sounds pretty clear cut. Have a read here about constructive dismissal. Unexpected demotions/paycuts are examples of constructive dismissal that are easiest to prove. Also in your DHs case he is (was) on a large salary, and another similar job will not be easy to come by, so compensation is more likely to be worth the effort.
To bring a claim he will have to put in a grievance first using the grievance procedure. It could actually be quite a straightforward one simply stating that he is bringing a grievance as this unwarranted demotion and pay cut are not changes to his contract that he agreed to, and that he considers them to be potentially constructive dismissal, and is raising a grievance accordingly. They will be obliged to hear his grievance and some kind of mutually less horrendous outcome could be negotiated.
His options are accepting nothing less than reinstatement on his previous terms and conditions, negotiating a settlement in return for his resignation and dropping a constructive dismissal case, or negotiating a different package to stay on, not as good as before but better than he has been put on now.
Which option is best depends on lots of things which we can't really tell here - realities of your personal situation, of the working environment, all those things.
I do recommend you talk to a solicitor specialising in employment law. Often you can get an initial consultation free, but as it is likely to result in some kind of negotiation, a solicitor would be best-placed to advise you and to negotiate on your DHs behalf.