What Causes A White Coating On The Tongue?

Hello I ve been experiencing a white coated tongue since January 2017 that just hasn t seem to go away. And I might add that my girlfriend is experiencing the same thing as well as my 4 year old son. All doctor s we ve seen have said it s looks like thrush but it was never confirmed. At 4 months after my exposure I had a routine blood and STD check which all came back negative as well as my son.Me and my girlfriend had a breakup back in November of 2016 and we both put ourselves at risk with having unprotected sex which was foolish. I ve been having symptoms that could be related to HIV but could also be related to things like a fungal infection or candida overgrowth. I tested negative for HIV at about 4 months as well as my girlfriend and she took the test 3 times. I just want to know can we put HIV behind us and explore other possibilities of what it could possibly be. And I might add that my son was on a inhaler in November 2016 which me and his mother used as well without rinsing our mouths out afterwards unaware that it could cause thrush. What is your opinion and thank you in advance.........and sorry for the typos kind of rushed typing this.