Thank you both so much!! I saw the first one first (of course), so I took out the bind off row (agh!
took me forever to find the yarn end to even start the process), put the yarn through like you suggested. When I tried it on him, it slipped over his head easily...
So then I did the Elizabeth Zimmerman bind off from the link you gave me (thank you for it!), and it worked like a charm. It's stretchy and fits over his head wonderfully now.
If anything, the neck looks a tiny bit too large now.. but I don't care! It fits, and I am not going to struggle with it anymore.
Thank you, Michelle, for the button suggestion too. I had considered if that might be necessary. I'm glad it wasn't for now, but I'll keep it in mind in the future.