I wouldn't panic too much about the lack of contract tbh. After 4 years in employment his terms and conditions will be pretty well established, his notice period is protected by law as is his holiday, both of which are the important things when it comes to redundancy.
Obviously not having a contract, or at least a written statement of certain basic information, is illegal, but I don't think drawing their attention to a lack of one is going to help anything particularly at the moment.
Have a read here about redundancy notice periods, it's one week for each year of employment in the absence of any extra provision. Usually it would probably be a month in a contract anyway, so little difference there.
Similarly, holiday entitlement is all set out in law, and even if his employer actually gives more than the basic statutory 5.6 weeks, the fact that your brother doesn't technically have it written in an individual contract doesn't make it any less part of his terms and conditions, the same as any other benefits.
The one thing a contract might say that might make a difference is something about pay in lieu of notice. If his employer do make him redundant and want to pay him in lieu of notice, in the absence of a contractual provision to that effect, they don't have the right to do that as such. It doesn't mean they can't, but what it does mean is that doing so would effectively be compensation for breach of contract, so payment for his losses incurred as a result of not being able to work his notice period. It wouldn't be much different, but would have to be tax-free and would have to take into account things like holiday he would have accrued during that 4 weeks had he been allowed to work them.
But rest assured, the fact that he doesn't have a written contract doesn't affect his rights in terms of redundancy consultation, or the ability to claim unfair dismissal should it not be genuine redundancy or not conducted properly.