This is *fairly* common. We did preschool, but then were sick at least one week a month the entire year of kindergarten (the difference between 20 family's germs and 500 family's germs). The first 3 months though we were sick one week for every two. And when I say WE, I mean WE. WE got sick about half the time our son got sick. Even though hubby and I were both at university, exposed to thousands of "kids", & I was working in a hospital, exposed to thousands of sick adults. ((Incidentally, we thought we'd get a "pass" on the Kindergarten Funk, because we were exposed to so many things, but no. We were sick as often as everyone else. Pfui.))
First grade is supposed to be better, but you still get sick a lot. We started homeschooling in 1st grade (so you might THINK we'd be less sick), but since we were still around tons and tons of kids in classes and sports and at the park, we were STILL sick a lot... because THEY were in school, and parents have been conditioned to keep sending sick kids to school.
While our elementary school was really tolerant of absences (because you spend the first 2 years catching everything in sundry)... they were NOT tolerant of being late. (Does that make sense? "We'd rather you were not here at all rather then here for most of the day?")
Anyhow, our illness level went down 3 months in when I finally said "screw the policy" and let my son sleep himself out. Several of the families in K started doing this, with full-on teacher support. (SHE had over 35 years experience, and was thrilled that we were willing to mess with OUR schedules, in order to accommodate our children's needs, and bring them in rested, well, and ready to learn. We all got nasty letters from the school and district, but the teacher shielded us from their threats of x,y,&z. God, she was AMAZING. Yay! Mrs. Salmon!!) We were on time about half the time...but some mornings he slept until 9 or 10am. Read 13-14 hours of sleep some days, but the normal 11 about half the time. I mean, think about it: kids are not only under huge amounts of stress adjusting to an entirely new system, but then they're also doing the growth spurt thing, AND fighting off or recovering from every bug they keep being exposed to. I also kept him home the first day or two after he was "better" ... since a lot of physical activity (hello recess) will cause a relapse (the adrenalin system shuts down our immune response, allowing a cold/flu on the way out to get a toehold again). As well as to keep from being exposed to another new "bug" while his immune system is still recovering from the last one
While it sounds like you guys have been heavily hit this fall, what with all the normal childhood illnesses (and since so many people aren't vaccinating, he may also be fighting off the preventable ones, like measles... even if we've been vaccinated against them, we still catch them, we are just able to fight them off before they knock us flat with full on illness)... you've also had to deal with Influenza A (which is ordinarily a 2 week down, and/or the swine flu varient which is 6-10 days). Hint: If he didn't stay home for the full 10days to 2 weeks, he isn't "sick again" but still sick.
Anyhow... while it sounds like you guys have been heavily hit, it's also a fairly normal Kindergarten/1st grade woe... so it's *probably* not leukemia or an autoimmune disease. That said, if you're worried it's something more severe, ALWAYS feel free to take him in to the doc, well or sick, to discuss your concerns.