>>>none of the staff have ever had contracts
Oh yes they have! If I offered to your shopping for a fiver, and you agreed to it, a contract would automatically exist between us. Similarly there is
always a contract between an employer and an employee, even if nothing is ever written down. Your partner has committed an offence by failing to provide each employee with a written 'statement of employment particulars' within two months of them starting work but that doesn't change their rights (and his obligations) under the legal contract which automatically exists between him and them.
>>>Would, (apart from his son) the staff be able to sue him for unfair dismissal if he said he was working on his own .
ALL of his employees, INCLUDING his son could go to an Employment Tribunal if he unfairly dismissed them. However redundancy (where it is simply impractical to continue employing someone because there is no work for them to do) does not constitute 'unfair dismissal'. Staff then have the right to receive redundancy payments instead.
If your partner is running the business as a limited company (so that the assets of the company are separate to his personal finances) he should consider letting the company fold (and then starting up in business, on his own account, again). The assets of the company should fund the redundancy payments of its staff where possible but, if there is insufficient money to do so, there is a Government fund which will ensure that the staff do not lose out.
>>>Also as the firm is VAT registered, would we have to set up a new company just for himself to avoid being VAT registered.
The requirement to register for VAT is determined by the turnover of the company (in relation to the supply of VAT-able goods and services within the UK), not by the number of staff. If the turnover will continue to exceed £77,000 per year, your partner's business MUST remain registered for VAT. If it no longer will be, he should seek to cancel the VAT registration:
http://www.hmrc.gov.uk/vat/managing/change/cancel.htm
Chris