Most Helpful Guy
I wouldn't recommend doing something super elaborate because he's a guy and not likely to appreciate it the way you might.
Have you read The 5 Love Languages? It's a great book and helps couples identify their preferred method of receiving love. Some people prefer praise, some prefer being touched, some preferred acts of kindness, some prefer receiving gifts, etc.
It's best to find out how he prefers to receive your love so that you can do that, instead of just doing what you prefer.
For example, I used to love leaving my girlfriends little notes around her bedroom and book bag for her to find later. But I didn't realize that she preferred touch (holding hands, hugs, massage, etc) and didn't really react as appreciative as I expected.
And so writing a song is pretty cute, but maybe first find out what he prefers just to save yourself the energy?
And congrats on being awesome enough to even think about it. He probably really appreciates you. :D
Most Helpful Girl
You said it! A song is a sweet idea. Or make a video that shows good moments of you two together.