K.,
for now, everyone in the scientific community believes this shot does not CAUSE autism. and by believe, i mean it has been tested time and time again. it's pretty much the only thing that has been tested so much for autism, really. you should know that autism signs usually begin appearing at 18 months, which is why so many people jump on the 'it's the vaccine that did it' bandwagon. it's so devastating, who wouldn't want something or someone to blame? people who have been naturally immunized also get autism, it's just not many people do that (or hadn't in the past, so data is limited - also it's hard to say "ha! my kid WOULD have gotten autism with the shot but didn't" we have no idea who will or won't get it.
here's another (of probably many that you'll receive on both sides of the issue) website to visit. my pediatrician had me start here to answer
http://www.immunizationinfo.org/
my thoughts: you got the shot, your other sons got the shot and everything turned out fine. the 'more information' that is out there now is HEAVILY weighted toward the 'not cause' versus the 'cause.' do research of course, but remember who will respond here, and when you look at websites - try to balance the blogs and articles with scientific research. we doctors aren't out for blood - we really really want your child to be safe and healthy.
all the best to you and your family.
also - you don't have to pay out of pocket for your son's shots. philadelphia has a program for free childhood immunizations, no matter what your income. i don't have time to search for the website, but it's out there. check it out. when you're ready, you can bring your son to that facility and get your MMR there.