I have read some of PPs posts, and all of yours @iamstillfuming, and you must be feeling very confused and upset (as well as quite rightly being fuming with your cousin and her partner), as you have never seen that side of your DS's "uncle" before.
I don't think anyone here is saying you were wrong to trust your close cousin's partner, as you and your family have known him for many years, and he is your DNieces father. However, many of us have witnessed over the years TV news reports, and reputable newspapers reports, about close friends and family members, abusing their child. It is almost unbelievable the amount of parents, step-parents, or other family members, who sexually abuse, one or more children in their own families. So many of them seem to have got away with it in the past, and there are probably, and tragically, just as many of them still abusing their children today.
From what I have heard and read, the abuser often wants - or says that they want - the abused child to feel "special" and "loved", and to not realise that what their daddy, or uncle, etc, is doing is wrong, and that they are "just" showing how much they love the child by giving them a very secret and special type of love. A love which unfortunately the other family members would not understand. The abuser would often tell the child something like 'if their mummy found out, then mummy would be either very sad and cry a lot, which the child obviously wouldn't want, or she would be very angry with him/her, her child.
In the majority of cases that do go to Court, it nearly always comes out that the other parent, or both parents if not involved, were absolutely convinced that the person found to be the abuser was completely trusted, and that it didn't even cross their minds that the accused could be the sort of person who would abuse any child, never mind her own. When they eventually find out that they were wrong, because the person they trusted, and possibly loved very much (eg child's father, step-father, grandfather, uncle etc) had indeed abused the child in such an evil way, they are obviously absolutely devastated and distraught that he could have done such things.
@iamstillfuming, I believe you that you believe that your DS's "uncle" would never harm him on purpose - hence you being so shocked when you found out that he had abandoned your DS in a strange, and potentially dangerous house - but you have already had proved to you that that is exactly what happened, so you already know that your cousin's partner is not who you thought he was. The only people who we know for sure about what goes on in their minds, is ourselves. We can never be 100% certain about anyone else, even though to function properly in life, we do have to trust that we are right about those closest to us in our lives, until something makes us start to wonder. Unfortunately, a partner of a family member, is never one that we should trust completely, unless we have really known them very well, all of our lives, and even then as I said before, we can never totally trust someone else. Please don't think that you cannot be deceived OP, as every single one of us can be.
Having said all that, with what little knowledge we now have of your situation, it is probably much more likely that your cousin's partner has been involved in either buying or selling drugs for a long time now. How unlucky would he - your cousin's partner - have been if the first time he was involved in, or the first time he took your DS to one of these "meetings", the shit hit the fan, and the police got involved? Most of us OP would want to deny that anything awful could have been happening to our own DC, unfortunately, none of us can afford to do so, not when we have our beautiful children to protect. I really, really hope that all of us who are smelling something stinking of gone off fish, are completely wrong, but I think that you really do need to contact the police again, and explain your worries. By the way, and sorry to be the bearer of potentially even more bad news, your refusal to give your cousin's partner's name to the police makes it far more likely that SS will be involved. If that man, I am fed-up of calling him "ycp", has done nothing wrong, and has nothing to hide, it won't matter in the slightest if you give the police his name, and if he has, then they should have his name...