The fact that he locked it changes the discussion. I'd help him buy a new one.
When I was 10 way back in the Stone Age of 1980, the Schwinn Scrambler BMX bike was the coveted bike to have. It was $180, which translates to $583 today. My parents said they'd pay for half if I earned the rest with my lawn-mowing money. (Imagine that today. A 10-year-old cutting lawns. Right.)
I biked everywhere -- to the supermarket to buy baseball cards, to one of the first video arcades (Again, imagine a 10-year-old today doing these things. They'd call child protective service) and dutifully locked my bike.
Two neighbor kids also received the same bikes, fully paid by parents. Not for birthdays, just part of their steady stream of getting whatever they wanted. Bikes were promptly stolen from video arcade as they forgot to lock them up. Parents immediately bought new ones.
These boys did not turn out well and while I imagine there were a number of factors, this episode was indicative of some questionable parental judgment.
Not really comparable to your situation, but it reminded me of it.