He has plenty of room above his head in the seat, at least 5 inches. I would say yes, definitely, if he was on the heavier side but he is only 23lbs so he'll probably outgrow rear facing in height way before he reaches 40lbs. For a child that doesn't need the extra 5lbs rear facing, I wouldn't get this seat. I would look more at the True Fit because of the taller shell. We bought the MyRide for my son when he was 22 months old, 33lbs, and around 37 inches tall. The pic in my siggy was taken soon after we bought the seat so you can see how much room he had. We bought the seat because he hit the max limit on his Marathon (only went to 33lbs rear facing). Now he is 27 months old and 35lbs so he has also hit the max limit on his True Fit (35lbs). He still has at least 4 inches of growth above his head in the True Fit though. Here are pics of him in the True Fit, taken today actually.
This is him in the MyRide, taken about 2 months ago. He's grown slightly since then
If you can afford it, I would look at the True Fit
ETA: I think the shoulder room looks fine. I guess what seat you should get depends on what your rear facing and harnessing goals are. If you want to rear face to about 2.5 yrs old, the MyRide would work but if you are looking to rear face past 3 yrs old, I think the True Fit would be a better choice. If he doesn't grow fast in the torso, he may make it to 3 yrs old rear facing in the MyRide. It is easier to install than the True fit too!