How and when should my boyfriend tell his son that I am the girlfriend, and mom has chose not to be involved?

I live with my boyfriend and son and have for the last year. I have known my boyfriend sons two months before he turned two. His mom has been in his life but lost custody four or so months ago, my boyfriend has had him for a year and some months, and mom has only had supervised visits. She decided not to show up to any court dates which easily allowed my boyfriend to gain full custody. After everything was done and over with she is now only allowed supervised visits but has chose not to hire anyone to supervise. Nor has she called for his birthday, we are starting to believe she is not going to be involved. My boyfriends son turned 3 in March, wondering when the right age we should explain this to him, and how we should explain?