Due to a fire in 1973 at NARA and he was in the Army then even filling out the forms may not get you any information either, 80% were destroyed.
You may be in luck with Utah state records though. Ancestry.com has WWI Draft Registration Cards online and R.Sterling Collett filed in Duchesne County June 5th 1917, he was living in Roosevelt and reported a birthdate of May 13, 1895, natural born in Vernal, Utah,USA, Farmer for himself in Roosevelt, single. To military experince question he states he is a Private in the what looks like Yarart or Zyart YUAC. I would think this is the Utah National Guard or Militia of the time. You can check the Utah State Archives for information, the history Utah's National Guard/Militia.
You can Google his name to seee if he is referenced in any online resource, biography, history. Check the local paper for 1917 as they had more local news of local folks and if his NG/Militia unit was called up for Federal service then there should be news of it published.