This is a complex area and one that requires professional advice, particularly if the amounts are substantial as you say. I presume the reason the question of switching to a PRSA arises at all is that he can avail of the Approved Retirement Fund options at retirement from a PRSA, which he cannot do in an Occupational Pension Scheme unless he is a shareholding director of the sponsoring employer.
A transfer to a PRSA will only be allowed if he has less than 15 years scheme membership and the scheme itself is wound up.