If you have already been exposed to the flu (which has happened, now that your son has it!) it is too late for you to get the shot. But yes, the National Multiple Sclerosis Society, among others, *strongly urges* folks with compromised immune systems --and people with MS have compromised immune systems -- to get the flu shot every year, and the pneumonia vaccine once every 10 years.
If you go to the National MS Society website -- www.nmss.org -- and search for immunizations, you'll find all their recommendations there. You might want to print it out and take it back to your doctor, to have on file for next year.
I hope you don't get the flu (just because you were exposed doesn't necessarily mean you'll get it!). If you do, do what you can to keep any fever under control. Generally a fever can cause your MS symptoms to flare temporarily, until the fever is reduced. I hope you stay well!
...I am not a doctor, nor health professional, and don't pretend to be one, here.....