I remember when I was in my early 20s, a gay friend of mine came to visit from out of town. He took one look at me and said, "I think we should go shopping." That day he took me to a store and showed me two color palettes: the brown/green world and the blue/grey world. It revolutionized my life. I had no previous concept that my outfits were clashing; it was like a veil was lifted and I've been able to shop for myself ever since. Your guy is likely in the same boat so take him out and show him how certain colors compliment others.
We don't know how to be creative with accessories
Most men think a tie is the be-all, end-all of accessories. Let your guy stand out from the suit-wearing, button-down, simple tie crowd by purchasing an accessory or two for him. Buy him one pocket square or tie clip and watch how he lights up at the compliments it gets. From there it's on him to invest in more, but don't be surprised if all of a sudden your guy has a stack of pocket squares matching his ties and is always looking dapper (also – tell him they need to match his shirt or tie).
We don't know shoes
In talking with many of my guy friends I've realized that we know about two types of shoes: dress shoes and play shoes. We don't often know, for example, the difference between oxfords and loafers. You're going to need to explain. We're quick learners. Also, tell your guy how to wear certain shoes with certain outfits. Tell him that it looks nice when his new red Pumas match his red flannel shirt. Most of us also don't realize that a suit could be more relaxed with play shoes or jeans could be fancy when paired with dress shoes.
We don't know how good it looks to match our shoes to our belt
...Or our socks to our shirt, or our shoes to our jacket, or any such combination thereof. Tell your man this simple piece of advice and he will already look one thousand times more put together at the next party you go to together.
We don't know who your favorite designer is
Help us return the favor by telling us (more than once) your favorite designer. That way when it's your birthday we know what to Google and, if we can't afford it, we can turn to your friend and tell her who you like so she can guide us to something similar. It's not easy to remember names like Proenza Schouler but once it's ingrained in our heads we have a framework within which to shop.