AVCs are associated with an occupational pension. If your husband does not have an occupational pension he cannot make AVCs.
He has a PRSA and he can choose whatever way he wants to make his contributions.
The main advantage of the monthly contributions are that they are probably deducted at source from his wages and he gets tax relief directly from each deduction.
An advantage of making an extra single premium payment at the end of the year is, assuming he makes this directly to the PRSA provider, that his employer has no knowledge of this extra payment. This can be to his advantage if he is seeking a cost of living pay increase from his employer. The employer does not know that he has enough spare income to increase his pension.
The limits for tax relief per year are the same for either AVCs or PRSAs.