I have to be honest... he now looks like a kid wearing a fake mustache.
I believe the main problem is how he looks overall as he doesn't look like a private investigator - at all. Most private investigators / detectives in cartoons look somewhat like this...
My recommendation would be to follow the trend, but put your own spin onto it. & make sure you give him a vice as all good detective always has some bad habit or other. Maybe he could constantly tug at his nuts, or smoke a pipe/cigar/cig like a chimney, or sneakily take drams from a whiskey flask or something every so often.