It's peach leaf curl. The tree needs to be sprayed before bud swell in spring. All you can do now is dispose of the affected leaves. Here is a link that might be useful: peach leaf curl

I'm not familiar with a red foliaged ornamental peach that the OP has, but my double flowering peach trees, which are pretty common, produce white fleshed peaches very late in the season, and can have quite large crops. Although the fruit can have a slight bitterness to it when not fully ripe, due to almond genetics, and they are somewhat small, they are actually pretty decent. I generally tend to not bother harvesting them because by that point in the season (late September) I'm generally sick of peaches anyway and have put up all I need.

Your question got posted in the wrong place and that's probably why you never got a response. Fertilizer is highly unlikely to help with the problem you describe and is probably more likely to complicate the issue. If you are still looking for answers, I would recommend posting into the main Trees Forum or into the Fruit and Orchards Forum, and I would definitely include a few pictures. You need to ID the problem before throwing a bunch of stuff on the tree and hoping for magic.

I have not used copper but hear it works well. I spray mine with a fungicide containing chlorothalonil and it works great! However, they must be sprayed while the tree is dormant and before the bud scales even swell and crack in spring. Too late to spray for this fungus once the leaves are out. I try to spray in March with great results. This year we hit some real warm weather in late February so I got the job done then.