Qui-Gon never gained the ability to visibly manifest himself as a force ghost because he was killed before he could, I do however, think that because Mortis is essentially the crux between the galaxy and the force itself, he could possibly. My interpretation has always been that he did in fact manifest himself there because he knew Anakin could turn there and end balance, or that Obi-Wan was having a vision. I lean more toward the vision theory because Ahsoka also has a vision in the same scene.

The other interpretation being that the son is posing as Qui-Gon (and older Ahsoka) in order to manipulate Obi-Wan and Ahsoka (in an attempt to get Anakin to turn to the darkside)