When our Sophie was diagnosed with lymphoma, the vet said it was a very aggressive grade of the disease and even with chemo, 6 months would probably be the high end of what we could expect. That was the main reason we chose not to go with chemo. She lasted about a month on steroid treatment and was happy and healthy up until the final couple of days. Every piece of advice I got was to go with your own heart and what you think is right. In our case, we couldn't see putting her through the chemo for an extra couple of months - for our sake. In your case, chemo could prolong his life for a much longer amount of time and it would be totally worth it. Only you will be able to know what is best.
Research, research, research as much as you possibly can so you can so you can have good conversation with your vet and bring up different possibilities and options. There is a wealth of information on the web about dog health issues and the folks on this board know what they're talking about.