His tail and feather will take a long time to grow... You just need to be patient.
As for the yellow coloring... That is very common, and all you need to do is bath him with warm, soapy water, and put on some sock whitening spray, then rinse that off.
If you don't have this spray, you can put a blue plastic bag in the water before you clean your horses feather... This gives the feather a very slight bluish shine, (You can't notice it unless you, look at the feather through a magnifying glass!!!) and makes the feather whiter :)