We cannot give you financial advice on this site - only a body regulated by the FSA can do that.
But the general principle would say that every time you move pension funds around, some smart arris takes a cut from the fund.
There is no harm in having several pension pots from different employments - they will all pay-out in the end when he decides to draw on them.
If he has very small sums in them, the annual management charges may be eating a bigger proportion of the fund - that is perhaps one way to consider whether to transfer them.
Also it may not be possible - depends on the trustees of the current scheme he is with.