One thing to know is that M. babies start getting sick at *least* once a month starting at 5-6mo of age if they're breastfed (formula babies sooner) because their brand new immune systems don't totally immunize them from the antibodies you gave them through your colostrum and milk. (It's also part of why immunizations for babies have boosters, there's no point in letting an infant die of the measles when there's a vaccine, but the vaccine won't protect them for life that young).
Also, fevers accompany teething, so fevers kick up BIG TIME, about a month before teeth start poking through.
Now some people *are* allergic to some components in some vaccines (like those who are allergic to eggs), and *M.* people feel somewhat ill after a vaccine because at least 50% of feeling "ill" is the HUGE amount of energy that your immune system uses in full gear, and the side effects thereof. (Your immune system makes your bones ache, shuts off your digestive system, spikes fevers... those are all signs of your *immune system* working, not the illness itself).
I'm not saying your son isn't having a reaction... but some people get so caught up in one area... that they forget to look for others. A classic example is that I thought my son was returning to the "horrible twos" phase we'd just climbed out of, so I was disciplining him like crazy. Only to realize he had molars coming in. Head slap. Ugh. Bad mommy moment. A little dose of tylenol and I had my love back. Poor kiddo couldn't help his temper, he was in pain, not being snarky/defiant/pushing boundaries. But because what was on my mind was the terrible 2's, what I was seeing was what I was expecting to see... and I didn't look for other options for some time. So DO be sure to consider other options, and to know that you *want* your immune system to kick in bigtime after a vaccination. It means you know for sure that it's "taking"; antibodies are being produced, and the blueprints stored.