Well, yes of course he does, they are his possessions, whether he bought them, they were given as gifts or he had had them for years. He has every right to retrieve them.
A call to 101 might help. Could his dad call? Could you?
Having said that, at 17 I came home from work one Tuesday night to find my parents had found me somewhere to live. I thought I lived with them, turned out I lived in a squalid bedsit. I thought I had books, records, record player. I thought my granddad had made me a desk and bench that had all sorts of secret things in, like 8 years worth of diaries! I thought I had jewellery, clothes, motorbike boots and jacket.
Sadly, like so many other things I had 'grown out of' or 'hadn't used in years' during my childhood, it turned out that I had none of those things. I had 2 black bin bags of stuff. DPs spent years telling everyone they paid my first months rent and deposit and gave me pots, pans, cutlery, crockery etc. 1 pan, 1 bowl, 1 plate, 1 cup, 1 knife, fork and spoon... no towels, no toiletries, no Post Office savings book (which had a whole £1.20 in it
), no teddy bear, that I had slept with since I was 3... nothing much at all.
When they moved to Spain, 25 years later, they made a big point of returning that teddy bear... I have no idea why they kept it, where they kept it or what I was supposed to say. DSis says they were disappointed with my reaction to it being returned!!!
Your DN will survive. His relationship with his DM may not. But you and his dad and the rest of his family can help fill that gap... I wish I had had family that did that for me. I was able to let go of the 'things' I have never managed to forgive or forget the total abandonment.