Being out in the Sun for ANY time at all, no matter if he's sweaty or not, will bleach his coat. The Sun bleaches everything. Don't like it, don't put him in the Sun. But I sincerely hope that you won't stall him just for aesthetic purposes. His owners could have kept him stalled, or he could have just shed his winter coat, or just grown his winter coat depending on when you got him.
There on the tips of fair fresh flowers feedeth he; How joyous his neigh,
there in the midst of sacred pollen hidden, all hidden he; how joyous his neigh