Assume you meant to post this in the freshwater fishing sub-forum instead of the fly fishing forum as it would be pretty difficult to fly fish from the bank. Jones Bridge, Island Ford, Settles Bridge, Paces Mill, Buford Dam, all good parks for trout fishing access...
Lots of wading fly fishing available just below the dam, but make sure you keep tabs on the dam release schedule. it gets VERY dangerous above Bowman's Island Shoals, enough so that PFD is required for safety even wading as I recall.
Closer in as mentioned Jones Bridge is likely the next best, but also VERY popular (read crowded) spot for fishing on foot.
Ha . . . when I think trout I think fly fishing. Good idea will post in fresh water section. The boys would be using ultra light spinning.
What park area is the best, if there is a best.
Main thing I want is to be able to fish without, repeat without, wading.
Years ago I used to fish below the Dam--mid sixties--there was no warning system in place or at least none was used and I had water rise on me when I was wading. Fortunately I had a boat I could climb in. I wrote the Core and not long after that a siren would sound whether because of my letter or others or just gained common sense I don't know, but none was sounded the day I was there.
Morgan Falls, Azalea Drive, 400 Bridge at Riverside, Holcomb Bridge.......
There are any number of places close to the city where he can catch a trout. Good luck!