I can tell you that this is an issue not only effecting your son but also effecting your whole family. I think I would start by explaining to your child that if you were to keep everything you ever purchased the house would be impossible to live in. I would then begin to explain how the "things" he has are not what is important in life. The relationships he has are what really matter. I would then set a time for him to choose 3-4 things to give to another home. For example if there is a preschool in the area maybe he could donate them to the school. If there isn't a school maybe a friend with a younger child would appreciate the gift. Once he starts parting with some small things the rest should be easy. Be sure to praise him as he gives things away. The praise is what will bring him to grips with getting rid of things. You can also try the one in and one out technique. For every one thing someone in the family brings in then they must dispose of (in some way) of one other item.
I hope this helps!
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