I can absolutely see why it’s taken a while to come to light.
Other child doesn’t pack boots by mistake - bloody kids.
Wears trainers home.
Missing boots not noticed when unpacking - because parents busy, or a bit stupid, or child told to unpack, or one parents only took out clothes for washing and other parent later sorted the rest, and half a case meant missing items weren’t obvious... there are lots of reasons.
One month later they’re needed. Realise not there.
Go to lost property at school.
The one WRONG boot prompted them to realise that her boots may have ended up with someone else. Their daughter remembers yours has the same ones.
No “swap” may have taken place. Your daughter may never have worn the currently held wrong boots on the trip - just grabbed them during packing.
Bottom line (subject to clarification on whether his text about size was an error) - you have his boots.
Your boot in lost property is a red herring, it’s nothing to do with you having his boots.
What if he had found a lone sock belonging to your daughter in lost property? It’s the same thing.
He doesn’t have your boots, has never had them.
If your daughter had brought home BOTH pairs, would you say “he took a month to ask, that’s too long, I have a younger child, I’m keeping them?”. I hope not!
They’re not yours, give them back.