In all likelihood the story is not true. Wilhelm II served with with several Garde regiments, but the uniform your great grandfather is wearing would indicate that he served as a private with a regular unit. To be honest, I cannot say which one; the shoulder board is a clue, but to my inexperienced eye not enough to make a definite statement. The original photo would likely have had the photographer's name and location; and the location usually would have corresponded to the soldier's garrison. Your great grandfather would have entered the service c. 1874, served several years, then became a reservist. It is difficult to trace the service of a regular soldier in the Prussian army, as many of the records were destroyed near the end of WWII. Any other information you have may be helpful in clarifying this; where he was born and may have lived before he emigrated; other photos, perhaps correspondence he wrote while in the service; he also would have been issued certain documents during his service which he may have retained, although I'm assuming you would have posted this as well if available.