I work with 2 - 11 year olds and believe me, when one kids gets something all the others do as well and sometimes it ends up right back at the child who infected them all to start with lol
When my youngest was 6 she had really had much time off 'ill', just the odd temperature here and there but during her year 1 she got everything going and even came down with pneumonia over Xmas but since then, touch wood, she has had a few days this year with tonsillitis but apart from that she went back to the odd day after that bout in yr 1
Make sure he is 100% before you send him back - if he has a temp in the evening, dont send him in the next day, I cant tell you how many times we send kids back home when parents have done this
If he needs Calpol/nurofen before school, he isnt fit enough for school - that's what we tell our parents anyway