If this is the case then what you are saying is true. If his muscles head at such an accelerated rate then basically he could bodybuild every single day and not over train. Body building is basically tearing out or destroying your muscle so that when resting it repairs and builds stronger in proportion to the effort it will be required to that. So Flash could indeed become very muscular with training.
The diet though, if he has a very high metabolism, he will have to eat a lot more to put on mass, if he eats balanced, he will have some muscles and stay lean, and if he eats less, he would basically be slimmer but super ripped.
So according to what you are saying, the actor playing Flash should be with decent mass but extremely shredded. Thankfully actors CAN pull off such a physique and have done so in the past.
Some examples are Henry Cavill in Immortals (not MOS lol, he's too muscly in that).
If these actors can get the correct physique, even the Flash actor should be able to. These days give an actor a good amount of time and a great trainer and he will mostly deliver.