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My husband hangs on to things, as do his parents and one of my children. My mother in law, for instance, keeps old calendars because eventually the yearly cycle will recur and she won't have to buy another. Never mind that people are giving away more calendars than you can use. I am sure it's nothing like the LW's experience, but I struggle daily to convince my husband (and my daughter) that much of what they hang onto is not only unimportant, but that it literally and metaphorically gets in the way of life.
I think Cary identified a reasonable plan for LW, and I would second finding a female therapist, though I suspect that there will be reasons soon enough for why the new therapist is deemed to be unsatisfactory. It's funny, but people who adhere to things disproportionately also seem to have weaker attachments to the humans in their lives. What the things represent to them, emotionally, has a stronger hold on their self-image and sense of well-being than their connection to loved ones. It either reminds them fondly of the past or of some future possibility that simply can't be discarded, as if discarding the thing is admitting the futility of future achievement.
As a practical matter I would strongly urge the LW to do what I have done in my own, much less cluttered house: Establish "safe" zones for him and his son where stuff may not be kept. In my house, this is the kitchen, the dining room and my side of the bedroom. LW should assert the right to rid the designated zones of anything being "stored" there. This would, hopefully, include his son's room and some amount of play space.