First, let’s remember that South American seasons are opposite of North American. September in North America is early Fall and down here it’s very early Spring. What this means is you are still getting snow and cold in many areas of Patagonia. North of the Ice Fields is very different than south, and north is more than likely okay to ride, but checking the weather for the high passes is a must because it can and has snowed in the high passes of the Carretera Austral. In late winter and early spring, snow and motorcycles do not always go hand in hand. It will more than likely still be raining and cold.
The start of the riding season in my opinion is beginning of November. The weather is still cold, and I have seen snow on sections of Ruta 40 that time of year but it’s generally passable by moto. I have seen riders as far south as Ushuaia in November however September is very early to try and make it all the way south and not run into any snow or ice. The high passes can have snow and the weather would be unpredictable at best.
My suggestion to this rider is to start his Patagonia ride in the north, around the Los Lagos Region of Chile and slowly ride south along the Carretera Austral. This will give him time to hit the southern pass into Argentina around November and eventually make it to Punta Arenas in late November. So, he gets September and October to explore the Los Lagos Region of Chile, which includes Chiloé Island, Futaleufú, Parque Pumalin to name a few. Then as the weather improves start making his way further south.
One thing I suggest to adventure riders is to always give Patagonia its own time for exploration and try to plan accordingly for the seasons. If you leave in Winter in the North, chances are you will end in Winter in the South! Try to get your seasons to line up where you are chasing summer not winter. Patagonia is huge and has lots to see and do, requiring time to enjoy and experience. The more time the better. While I love the fighting spirit of many Adventure Riders, I have to say a little planning goes a long way.