What do you suggest if my father-in-law was told he has a tumor (6cm) on his adrenal gland?

Work up: Hi. That's a big adrenal tumor, and needs prompt evaluation and treatment. Some of these tumors make hormones that can harm health (e.g., cortisol, aldosterone, epinephrine and/or norepinephrine and/or dopamine, etc), and some are cancers, and some are both (i.e., hormone producing cancers). That's a big tumor. See an experienced endocrinologist for diagnosis & planning.