Husband and wife divorced - one child. Husband dies - no will. Husbands family move in and remove items from house etc. surely child is next of kin and deceased family have no right to interfere!
If the child is under 18, then yes they do have a right (or at least a claim to a right) to get involved - but on behalf of the child, not for their own benefit.
If the child is over 18 then they cannot get involved unless the child requests it.
If there is no will then letters of administration have to be applied for to the Probate registry, until an administrator has been appointed by the court then nothing can be distributed, paid, inherited etc.
The boy needs a legal guardian which would be his mother and she should apply to the probate Registry for the role of administrator i would imagine. The family may feel they have a right but in law they are committing a crime.