I have a boxer that is 12yrs old he developed a lump under his chest which was the shape of an egg but it has grown a bit larger and he has lost a lot of weight you can see his ribs and he has gone really thin around the hip area he is eating really well but always seems to be hungry what could be wrong

The lump could be a lipoma (fatty tumor) or something more worrying like a malignant mass.
Boxers are a breed that is predisposed to cancers.
Not trying to worry you, but with the weight loss despite being hungry constantly, and his age, it is best that you get him to the vet to be checked out.
Good Luck, let us know how it goes.