My brother has lymphoma. What should I tell him?

Mostly curable. There are two main types of lymphoma, hodgkins and non hodgkins. Most of them are highly curable and treatment mostly consists of chemotherapy and radiation. The exact prognosis depends on the stage, the pathology and size of lymphoma. For most cases we can be optimistic about outcome.
Be supportive. Be supportive and helpful in his treatment and recovery journey. There are different types of lymphoma- some with excellent chances of cure and some with reasonable chances of improving survival at the very least. Newer treatment options are coming out with regularity and outcomes continue to improve. Treatments depend on the type of lymphoma but most patients are able to undergo treatment fully.