It sounds like you've got yourself a real smart one there! I used to pull a lot of tricks to try and get out of school/class when i was younger, but usually just saying I had a stomach ache/fever. Once in a great while I might have pulled something bigger and said i sprained my ankle, etc.
I'm not sure if you've tried this yet, but what really worked and got me not to fake ill was being told that I wouldn't be able to do any of the special activities I enjoyed for as long as I was "sick".
Sometimes with a smart/tricky child, you have to be tricky with them as well. Next time he says his stomach hurts & he can't go to school, feign pity and tell him something such as - "Oh no, well this is just horrible! I was going to take you and your little brother for ______ after school, but I guess if you're too sick to go to school, I'll just take your brother by himself."
And the same formula for the various other injuries. In most cases, if it's really something the child enjoys, they will cave and go ahead and go to school. If not, well then you have to do what my parents always did. Go ahead and let them stay home, but make sure the day is not some kind of vacation. If they are "sick", they should stay in bed most of the day and rest. No video games, not a lot of tv, etc. Trust me, being confined to their room and being bored all day will get old very quick!