Bless you Staciewells1 for adopting a sweet senior soul. I have gone this route three times and don't regret a second of it!!!
We have four furkids here so free feeding is out of the question. They eat in the morning and the evening. Thought free feeding is certainly not a bad thing, designated feeding times will associate you with food and could speed up establishing your position in his life. Our last senior girl, Zoey, was clearly free fed before we got her. The first morning she never got up...that was the one and only time
She figured out fast that eating time was a good thing.
As far as grass walking goes, it doesn't say where you live, but I have had two terriers who do NOT like wet paws so if there is dew or rain on the grass that might be what he is objecting to
Incredibly, they both LOVED snow!!!
My seniors were all neglected dogs (though not abused) and after only a few days on consistency with eating and walking they got their confidence.
Good luck...you sound like a very loving responsible owner so this little one should be thriving soon. Have you posted pictures anywhere???? Would love to see him. What is his name?
Good luck at the vet too...some high quality kibble should take care of any skin problems he has and a good grooming will make him feel good as gold!