What is this lumps on my dogs chest?

My 8 year old pit bull/mastiff mix, chewy, has a lump right next to his right arm-pit. They've been there since last december. I am very worried about it. He seems fine, his behavior hasnt changed. I just dont know what to do. /: The Vet didnt seem to be to worried about it. But it looks like it has somewhat grown. Please help me!