How do Du explain to a 7 yr old little boy that some people are just mean?

My son was at a birthdayparty and someone took his Transformer jacke it Had bumble bee on the back n we cant find one like it. It wasnt taken Von mistake it was the only one there like that plus his name was on the inside. I got him a new mantel but hes still herz broken and thinks who ever took it will bring it back. the parent of the little girl who was throwing the party says she thinks she know who has it but cant prove it.
How do I cheer him up?When he is so sad