Were you renting to your son? Did he regularly give you rental payments? If so, Google your state's laws on abandoned personal property by tenants. You have a legal duty to secure (temporarily) a tenant's abandoned property. Each state has different rules on how long you have to store and how you go about destroying it.
If your son was merely a guest you have no legal duty to protect his property. A guest in that he stayed in your home without paying you a dime. That property can be removed easily. In fact, if you box up the bulk of it, the police will normally take possession of it as, found property. Not always, but usually, call them and see what they say.