Steph,
I wouldn't worry too much about this. Every kid, at one time or another, fakes sick. Adults need a day off once in a while, kids do too. I have a 12 year old on the autism spectrum, and he will fake sick when life at school gets to be too much. Staying home sick means you stay in bed all day. He has no TV in his room, only books and a few toys. He will sleep all morning, but by lunch time will fess us to just needing a day off. He said he was sick, he still stays in bed all day. That is usually punishment enough.
I would use this as an opportunity to talk to your son about school, find out if there is an issue with another child that is causing your son to feel stress. Sometimes even the best teachers aren't aware of issues such as this. Assure your son that you understand that sometimes everyone needs a break, but follow it with the expectation that you deserve respect because you are the parent, and thay you expect him to be honest with you. I have found that when my kids know that they can tell me they need a break, and that I will really listen to their concerns, they will just tell me they need a day off and not fake sick. It only happens about once a semester, sometimes only once a year, we can usually work out a deal to send them to school, even on those days they really would rather stay home.
Look at the lie for what your son needed to accomplish. I don't allow lying, there are worse punishments for lying that for telling the truth. He fessed up, as they usually do in these cases. The lie lasted a few hours, did not harm anyone or anything and in the grand scheme of things, was quite minor. Punishment for the lie should fit the crime, something simple that is over and done in a day. If it happens again, you will know that there is something going on at school that he is trying to avoid.
The most responsible kids need a day now and again, it just cannot become habit.
Best of luck,
G.