Ok... so for the party, make it a modified version of a Pinkie Pie Party! I don't know how old you guys are so I won't be giving you any ideas, but I'm sure you can find the proper modifications. Its always fun to have group activities at a party!
As far as the gift goes, if you're wanting something strictly pony-related then a plushie is a very safe bet. While I don't own one myself, they are a very popular item (and I kinda wish I had one lol). I must warn you however, that you have to be careful not to over-use his interest in MLP. I'm sure he's into other things so maybe find an activity or a food choice that caters to his other interests? Or even better if you can find a way to combine them! If you over-use a person's interest in one thing they tend to think that you view them as one-dimensional (which causes unnecessary strain on relationships).
Now, with the old man lecture out of the way, you could also consider getting him something that he could carry with him. Depending on how discrete he normally is about it, you have a variety of options. For instance, if he doesn't advertise the fact that he's a brony in public (like me) then don't get him a t-shirt. He won't wear it. But you could get him a pin! Pins can be easily hidden away when not needed and still carried around as a constant reminder!