when you say "held in trust for mother" - do you mean that she has a life interest in the business and then to someone else on her death?
Unless the son holds a power of appointment and has exercised this by will, no his surviving spouse does not inherit his trust duties. Just because the son was trustee it gives him no beneficial interest to dispose of the property as he wishes - subject to the terms of the trust. However, this is terribly complicated and depends on who left a will and who died intestate (ie the chain of executorship can only pass through testacy and not intestacies). Looks likely that you will need to invoke the Trustee Act 1925 and appoint a new trustee.
HOWEVER - if you post full details of trust document leaving the business shares in trust to mum (whether a will or trust deed and any substitutional provisions) and whether son died testate or intestate I may be able to help further.