100% you're being trolled. Just ignore him. He's trying to say, "You should look at your graduation banquet as a great honor, so you should dress really well in light of that, and you shouldn't be told what to do, I'm not trolling you I'm just being nice to you, I won't tell you what to do, you should dress however YOU think is appropriate, I'm not telling you what to do but...."
If you think too hard about your outfit, you're doing it wrong. The outfit looks good.