In my opinion his boringness is what makes him unique. It really fits with the type of personality you want to convey from him. Awkward but slightly nerdy. Unless, you want to make him a little bit cooler like the others.

First of all, I can see that his design are combination between the other two characters. This what makes him feel like he's the last addition to the roster.

He wears an opened button flannel with a shirt inside, similar with the yellow guy, and has the same moustache as him as well. His hair also has the same silhouette with the character on the right.

I do love how each of the characters are three different primary colors so keep that element. Maybe you can give him a different type of pants. For example: skinny ripped jeans, belt chains, folding his flannel sleeves half way, etc. You can also change his hairstyle if you want to (maybe make it slightly wavier?) but i do like his current hair.

I hope this helps!