Whatever you do, do not show him the brony threads on this forum.
Don't get after him too much for it. If he thinks you or anyone else is really bothered by it he'll make it an even bigger part of his identity just to test the limits.
The brony friends I have sometimes say they like it because it reminds them of old school cartoons that aren't around anymore. Maybe introduce him to something actually old school and more masculine, like G1 transformers or something, to try and subtly get him off it.