Hi , I'm James DVM MRCVS and would like to help you with your question
firstly it's very nice that you care and I thank you for that
By the sound of it, it might have an infection in his jaw , either as a result of his teeth or of other origin.
I would try and take them to my local vet . tell them your story - they will examine him and tell you what it is that is causing this selling .
I found that in a clinic there is always someone willing to take a hamster (one of the staff ) as they require little care . especially people with kids .
So you might land him a new home and the best care ever
it is very sad that people throw away animals which are sick or too much to handle
in the meanwhile - make sure he is drinking
take a small syringe or a pipette and hydrate him with water that we mixed with just a sparkle of sugar as he/she might be too painful to eat anything else.
Make sure to take him to your vets this morning as all regular vets are still open .
Keep him warm and dry.
I wish you the best of luck.
Please let me know if you need my help further .
All the best,