You can sort of see his arm in this shot of Breloom fighting May's Combusken.
And it's fully outstretched on this Pokémon card.
I couldn't find a gif of it, but if you have caught a Breloom in ORAS, go to its PokéDex page. Select the icon with the roaring Pokémon and keep pressing A. Eventually it will show Breloom's punching animation, which shows its arms stretching outwards.