Firstly, in your question you stipulate that you only have one staff member, if this is the case then it should already be apparent to your workforce that the business is experiencing difficulties, i.e. the phone isn’t ringing and orders aren’t coming in at their usual high rate. If this is the case, informing the staff member that changes need to be made will come as no surprise.
Many employees may well be prepared to reduce their hours of work or reduce their wage if the only other alternative is to lose their jobs completely so there may well be no opposition to the change. This way you can avoid a redundancy process, and if your son is willing you could still employ him on a voluntary basis to help shut down your shop.
Alternatively if your employee isn’t willing to reduce his wage and redundancy is the only option, then individual consultation with the employee should be carried out, and although there is no statutory length of the consultation period, those set out in any contractual redundancy policy or trade union agreement should be followed.
Ultimately, it is open to you to terminate the employment and simultaneously arrange for a short-term contract for your son. This should be your last resort and all efforts should be made to agree terms with your employee because this is still a dismissal in the eyes of the law. However, if you have a solid business reason for making the changes, then you may well succeed in arguing the dismissal was fair.