My dog has bad breathing problems, help please.?

My dog is a 14 year old mutt, We arnt sure if its food because it starts up after dinner usually, he has a tumor like bump on his chest, but we dont now what it is, so its possibly cancer or allergies, maby an asthma attack, but maby u guys have a better guess at it. i know hes old but i can't bear to let him go, hes my best friend, always has been, so please dont say, "put him down" or "it's his time" because its not.