Looks as though all which could be done under the circumstances has been done. Sorry your son had his property stolen. Was bike padlocked? It should not matter, but the question will probably come up.
**I never heard of “Offerup” before today.
It might be another site similar to Craigslist, but less crowded.
I bought my 75 gallon fish tank with a stand on OfferUp in 2020. It was 2 years old and I got it for $200, which is a steal since stands cost $250 alone.