Try find fertilizer with all macro ments and micro ments included in mix this called a complete fertilizer.

Another caution water lightly with bottle water are rain water. Tap water that tread could cause problem I'm seeing in photo. Peach is one easiest to drown.

Your potting mix could have a drainage problem. A soggy mix can cause foliage that looks like yours. Al

The sprouts at the base of the tree are root stock.
Won't produce the same fruit your Dad's tree does.
Toss the sprouts and by a named kind

Looks like spider mites.

I second plumhillfarm, the speckling on the leaves could easily be spider mite damage. The other observations are good as well.

Hey, thanks everyone! I tried two or three things at once from all of your suggestions and I don't know exactly what it was that saved it but it's decay has stopped and it's growing new fresh green leaves again!

All the best!