Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Where on his chest is this located? Up by his throat or lower?
Did his vet sample the fluid in the mass to confirm their suspicion?
Any history of trauma in that area?
Can you take a breathing rate for me (just count his breaths for 10 seconds + multiply by 6)?
Are his gums pink or pale/white?
Can you take a photo of this? If you can do so, this will let me see what you are seeing. To post them online, you can use the paperclip icon on the tool bar above your text box. Or if you can't see that on your computer/phone, then you can post them on any other site (ie Flickr, Photobucket, imgur, etc) and paste the web link here for me to check.