i went through this same EXACT situation last summer . only ... i was at the other end of it
my parents had a stirct 'no texting' policy for me and my two brothers (we're on a family plan) but, i figured, 'hey, theyre only 10 cents each. why not text just a little?' not smart at all ! i did text a little, but then it became habbit, and i did it without even thinking first. my parents got the bill and my texting was wellllllll over $650. my brothers (18 and 20 at the time) just texted a little. soo ... i got grounded for what was supposed to be 2 weeks, but ended up being 20 minutes, ending with an unlimited texting plan for the whole family ....
definitely not a good example in this situation, but i do
not think you are being too harsh. way to go =] teach 'em a lesson. he might 'hate' you for it, but he'll learn. sometimes i wish my parents were more enforcing ...