What I saw in the video (I only watched the first minute or two), was a dog that may have been bored or uncertain (he seemed to be pacing a bit), and wanted to go outside... and when told "NO"... was, like "But... Ma???? I want to go outside!"... after the second "No", he did turn around and start to come back to you - at which point, I would have given him a "good boy" and also a "come here and lay down". Then, if he did as you asked, LOTS of praise.
He's new to your house, he's trying what used to work... and now it's not working... he needs guidance one what you WANT not just what you DON'T want. Having a crate and some beds around the house that you can point to and say "go lay down" is a great thing. I would also be giving lots of praise, pets and/or treats to let him know when he's doing something you LIKE. SO... if he's just laying there doing nothing, give him a calm "good boy" to let him now he's doing good by doing nothing. Basically random acts of kindness throughout the day to let him know he's home now, and he's in a good place. Even if the rules about barking and being outside have changed.
You have to teach him the new rules in a firm, but kind and generous manner. Just saying "NO" all the time will get very frustrating for BOTH of you (I think he recognized your "no" as you said it, your tone and energy seemed right on point, to me). Find something you can say "yes" to, and see if you can build on that.
I'd also make sure he is getting enough outside time and exercise. He might need more during his first few weeks or months with you, as he's getting to learn his new routines.
And... Kudos for taking in an older dog with medical problems!! He looks like a calm, sweet dog. I hope you have a many happy years together!