Yes to the sling walking. Keep the sling under him for a couple of weeks yet.
You can gradually loosen it, so it is under him, but not actually supporting him, but there if he starts to slip.
I posted a sticky on the fine art of holding a sling and collar while pottying
p.s. Buddy looks wonderful. He is doing so well. It makes my heart happy. I can't believe it is only 3 weeks and he is already doing that great.