Nagnacarta - first of all my deepest sympathies. It is not nice to be facing this kind of thing at all.
I do know a few DHs who are facing this and it is very very hard for them and their families. The first thing I would say is that sharing your fears is better than bottling them up. Your DH will know you are worried so talking and sharing rather than holding it all in is better. You will not be adding to his burden. Get him to talk as well - you know what us blokes are like. All strong and resiliant on the outside but jelly in the middle.
Having a crisis plan if the worst comes to the worst is also a good thing for YOU to work on as your DH is so busy. It is positive - it gives you power over events rather than letting events drag you along.
Redundancy cover will probably not apply if DH gets made redundant straight away but stil worth exploring. Usually there is a time delay before it kicks in as others have said. However, maybe you are covered already on an existing house insurance policy or even a life insurance or health cover policy. Check all the policies you have very carefully.
Try and work out a crisis budget. What is the bare minimum your family could live on? How much extra work could you do if DH was a home. Could you take a second job. Is there something part time he could do - even low grade low pay stuff in a crisis? Try and get a short CV together for him so he can get going on looking for a job straight away if he does lose his current job.
I suggest you try to work out how much redundancy pay DH is likely to get. Go and talk to the DSS and JobCentre now to work out benefits, tax credits and so on that you may be entitled to. The social security system is very complex and you will need time to learn the ropes. Not sure what element of mortgage is paid and in what timescale. Best to know before it happens.
Do you have a right to delay payments on your mortgage for a while?. Ask the mortgage company what the are doing to assist people who are made redundant?
Could you sell your house or rent it out and move in with relatives? This is obviously a crisis measure but it is worth exploring before you really need to.
I hope very much it does not happen. Firms tend to try and keep core senior staff as long as possible so they can kick start the business again once things pick up.
Hope that my post gives you some thngs to think about. My best wishes and hope it all works out OK.