Sandy V. said:
This is SO true. Do not, I repeat, DO NOT back down!!!
I think the punishment is very reasonable and I would not lift one finger to justify it to him other than: "Your actions; our consequences".
3 D's and 2 F's is quite unacceptable for a bright student. Your expectations were set, the consequences if he failed to meet them were spelled out, and you routinely asked him about his assignments and he deliberately misled you. IMO, he should have a period of at least a week without any of these items you took away so the "shock" sets in, and you could put some sort of incentive program in place so that he can start earning them back a little at a time. Not all at once, though.
Good luck - as my tag says, parenting is NOT for sissies!!
ETA - I DO believe in punishment for bad schoolwork, especially when it is obvious that the kid has the "smarts" to do well, but they're simply too lazy to complete assignments or, even worse, to turn in the completed homework sitting in their backpacks!! At age 14, school is your JOB and there is NO excuse for not trying your best.