The problem is that legally it is none of his business.
Morally he may want to know who his child is spending time with, but this is literally one of the impacts of a couple separating, that child's life essentially gets split in half, with each parent having their own family and friends etc.
And it goes way beyond just the introduction of new partners, but we focus on new partners because they potentially stay over in the house. But the couple will have different friends, will have extended family who they may go and stay with for instance, the new partner may have children of their own and those children may be adults with their own partners and extended families. The partner's children may have friends over to play and consequently their parents may come over, the list is endless.
So while in some ways it's normal to have some concern about a new partner, I think that we need to be honest here and say that the majority of people who feel that it's their right to meet other people their child comes into contact with and that that right should only applies to new partners most likely do have their own (albeit probably sub-conscious) agenda and reasons for wanting that.
Because it is literally not possible to demand to meet everyone the child comes into contact with during access visits, and any one of those people could be of equal risk to the child.
He has to trust his ex at this stage to do the best by her child, and while I don't agree with double standards which appear to have been applied here, what's done is done and in the future he may well wish to keep his cards closer to his chest.
If there were previous concerns these should have been raised long before a new partner entered the scene. If not, why not?