Can you sit down and figure out what clothing they've kept? Write it down as best you can from memory and keep it as an 'inventory'. Then next pick up, you send no clothing (or just what's needed) and a polite note with "You have XX shirts, XX shorts/trousers, XX pjs, XX jacket/sweater of DS's that have not been returned from prior overnights. You therefore have sufficient clothing in your possession for DS's overnight stays'. I'd actually probably go so far as to describe the items. You aren't asking for the 'things they bought'
back, you're simply saying they have enough clothing for DS at their place. If you do send anything put in the note 'please return (describe clothing you're sending) when you bring DS home on (day)'.
I'd also send DS in neat, clean 'play clothes' rather than a 'nice outfit'. I'd be willing to bet they return him in the exact same clothing you send him in. And I'd continue to send him in those same clothes each time.
Just write a note in the book with the date and 'requested information not provided, XX refused to speak to me'. Brief and factual. And continue to send the book, annotated for each time this happens.
At this point neither he nor his parents have to do the book nor speak to you. Until there is a court order signed by a judge it's all just 'requests' sent on your behalf by your lawyer. But as long as you are polite and reasonable in your requests (which you are), it will NOT look well for him in the judge's eyes if they aren't at least doing a minimum bit of cooperating.
I think it's a bit of giving him/them enough rope, iyswim. Patience, patience. I think if you just bide your time and bite your tongue you'll be pleased with the results in the end!