Basically, if you have all the requirements and all set to marry him go and propose to him when he comes to the valley. When you do, he'll turn you down. No matter how many times you try to make him take your feather. So, you go home and make sure you keep your blue feather in your bag and go to sleep. When you wake up, there should be a note on the table from him. Continue day as normal, and when your done with day things, go to bed and then you'll wake up since Skye is coming in. Then propose to him. You'll know which one to pick but I'm not including any spoilers.
But for me, all i had to do was sleep with the blue feather in my bag and he left the note on the table without me having to show him the feather when he comes to the valley but turn you down.
You can marry him yes, but to propose to him you do it a tad differently. He is a special bachelor after all