Presumably your husband is leaving about to leave employment before the end of this year.
First thing is to maximise his tax free extraction from the Company.
Second easy as pie to register for SE tax.
1. STR form if he believes income is going to be below €127k; Otherwise TR1 form. Revenue help line will assist. Return to his own tax office.
2. Register for VAT if he feels that he is going into a business where he ought to charge VAT; but he does not need to register if his turnover wont exceed €25k to the end of the year or a full year. Some businesses (eg mortgage adviser ..is not a vattable business) many are if they are services or product.
3. If all his clients are VAT registered, it means they can claim the VAT back and whether he registers or not wont bother them. If they are individuals like houseowners, then he will be charging more and having to account for VAT.
Hope this is helpful