Hi and welcome to the forums at Car-Seat.org.
It is only rf'ing that the 1" of room above the head applies. In terms of the 1" rule when rf'ing, you would measure by sliding your hand straight along the top of your son's head until you came to the back of the car seat, then you'd look at how much shell was above your hand from the point it touched the car seat to the top of the shell. (your hand should be flat and touching the entire length of his head if you slide it the right way - it should end up being at a right angle to the shell of the car seat pretty much.)
For forward-facing, the My Ride is outgrown when any one of the following happen:
- child reaches 65lbs
- tops of shoulders go above the top harness slots (check shell and not just holes in the cover)
- tips of ears go above the top of the shell (measure parallel to ground in this instance.)
I'm not sure of your son's age and weight right now, but if he is still young, I'd encourage you to look around the site a little bit and gather some information on extended rear-facing.
The minimum to turn forward-facing in the My Ride is 1yr old, 22lbs, and walking unassisted. Best practice is to keep your son rear-facing as long as he fits the seat within both the height and weight limits, and as long as it works for your family. I highly encourage parents to aim for at least age 2 provided the seat still fits because we have the most data up to that age point.
I hope this answers your question and if you need more clarification please feel free to ask.