There is a Robin that's been flying headfirst into ...

Put things in the window, he just can't see the glass and thinks it is an opening that he can fly through.

Use suction cups and hang sun catchers, stained glass, ribbon, bows... or put a few lines of masking tape across the window... just something to let him know it is not a way into your home.

Of course, another thing is, he could be seeing his reflection and trying to fight that 'other bird', but it's more likely the first.

At this time of year it is most likely a male Robin trying to attack his own reflection in the glass. They are very territorial during the nesting season, as are many other birds. Some more than others. IE. Mockingbirds. If he were just trying to fly thru the window, he would probably have already broken his neck. The attack flight is slower and at the end he strikes the glass with his feet and beak rather than a straight on full flight that would cause his head to hit first at a higher speed. I really don't what you can do to do away with the reflection in the glass. Short of covering the glass someway. I noticed AwlArtist has some good suggestions.