It is cancer however this type is known to have caused no problem with recovering (Depending on the type).....
You need to address these questions to his doctor. And if you still aren't comfortable with the answers you get, ask more questions.
This depends upon whether this is diagnosed as Hodgekins Lymphoma or non-Hodgekins. What is his age and is he undergoing chemotherapy and/or radiation treatments? The answer to these questions really determine the correct answer.