I'm backing up everything you've been told re getting together payslips, handbook, contract and any notes or emails he has re the extended hours and taking them to the Union and to ACAS.
If he really has been working the same pattern consistently for 2 years, then, yes, it can be treated as a contractual clause, and its not (necessarily) as simple as just saying, 'right, that's it, we're changing!'.
For a start, unless they can roll all affected staff onto the same or better hours/conditions in another reasonably suitable role, then they're potentially looking at a redundancy scenario. Whether that helps you depends on the answers to the below.
How long has he worked there in total?
How long has he been in his current Supervisor role?
When, precisely, did he start working the extended hours, and has this been continuous, with no changes of start/finish times?
Is there anything in writing about this, or how long it was going to last?
It might be , with such a high staff turnover, that management hadn't realised that there was anyone with 2 yrs + service, much less 2 yrs+ in an affected role, and so they haven't factored his position into the changes. Get the stuff together, get advice and start talking to them.
In regards to one of your later comments, they'd be on very thin ice if they cut staff and cut hours for existing staff, and then start recruiting new people in to meet the increased demand on the day shift. If the volume of work is there, they should be looking at offering that work to existing staff LONG before they even consider recruiting.
But, ultimately, if they want the night shift gone, it'll happen, so the goal now is the best alternative same-or-better role they can offer, or a compromise you can afford. And job hunting!
In the short-term, things you can do:
Check your benefits entitlement with CAB.
Check his Tax Code is correct, (it sounds a bit off with the take home figures you've given - or there's something funky about his pay).
If you're married (i'm guessing not from DP rather than DH) Check that you've transferred the maximum allowed tax free amount of your code over to him, as you are not using it.
If he wears a uniform, check he has declared this to HMRC, too.
Start talking now to your creditors and you Landlord if you do think there's going to be a problem - the earlier, the better - and get your financial statements ready, as well as reviewing what order to start cutting back. Remember, Rent should be the priority bill above everything but food and travel to work, then heating (you have two very small children, otherwise this would be the other way round, but minimise this as much as you can now it's coming into summer), then council tax, then water, then unsecured debt (you mention a loan?) only if you have spare money.