No he wasn’t. Ending a sentence with a preposition is perfectly acceptable these days. It’s a stupid rule to begin with and has never had any basis in actual written or spoken English.
Anyway, START, you’re just a kid. It’s no big deal to be a little sloppy with your spoken English, but (like my high school wrestling coach like to scream) you practice like you play. Letting your habits get the better of you could cause problems later on down the road.
Is bad grammar offensive? Not terribly, no. But it does say a lot about who you are and where you’re coming from. A firm grasp of the language is one of the best ways to make a good impression, whether we’re talking about a job interview or an email.