They appear fine on my desktop. How is your boss viewing them? If you sent your boss a link via say, facebook, that will display in a square format and will cut out the sides. Also, sometimes when viewing a series of thumbnails in an end screen once the video is done, those thumbnails will be slightly off. but they appear to be widescreen on your page. Unless you mean there's meant to be even more image to the right?
UNLESS you're talking about your YouTube BANNER. In which case, if it's viewed on mobile, it WILL cut off some of the sides as that's just how YouTube crops it for mobile devices, in which case you need to design the banner to fit your important elements that you don't want cut off in mobile, closer to the middle.
But if it's just thumbnails, they look fine and it's probably the fact you're sharing videos through social media which will sometimes display in square format rather than the regular youtube thumbnail format. That's just how it goes and I wouldn't worry too much about that.
