In the uponayana ceremony, the boys should take 3 steps. Why the boys are stopped at 2.5 steps to remains at home? Is that mean we are not following the actual procedure?
Upanayana means the initiation of studies in the gurukulam after getting the gayAtri mantra upadesham.
The child after the ceremony proceeds straight to the gurukulam for the studies of shastra.
Traditionally these boys were sent to ashrams with the gurus to learn in a gurukul system of education but in modern times this act of the boy doing for first alms round in town and leaving his family for gurus hermitage is done symbolically within the family and is later stopped by his maternal uncle (Mama) from leaving. Source
This was the practice followed in the ancient days. However, in today's time most of us do not go to Gurukula. Hence the maternal uncle usually stops the child once he commences his journey towards the gurukula.
This has become a custom that the child should be stopped else he will proceed to the gurukula.