It just looks like his coloring/pattern to me. If it has spread for over two months than most likely his color is still developing on him, and many times bettas get more colorful or change colors once they live in an appropriate, stable environment...this is GOOD! If none of the other fish show signs of illness than most likely your betta is healthy. Ich spreads fast and the other fish would have definitely been affected by it by now. Just FYI, Ich is easy to get rid of even without any medications. Just raise the temp to around 86 degrees and keep it there for 2 weeks. The ich will die off eventually. I think you have nothing to worry about at this point though.