I would start using them as soon as possible to get your son used to the idea of wearing them. I think the longer you wait, the more resistance you may get from your son...possibly when he needs to wear them the most. It seems like my son was about 4 when he started wearing them. Now that he's not walking, we're not as faithful about putting them on every night, but he will still ask to put them on occasionally.
As a mum of a little boy who was diagnosed late (5 1/2 and already had contractures; stopped walking at 7) I firmly believe in the benefits of getting the boys into AFO's early.
Jennifer and Patty were spot on when they advocated familiarisation with them prior to the need arising - even if they are only worn for a couple of hours each day / week.
Over the years we have noticed that by the time the contractures develop the boys are really starting to slow down, and as such they are having to deal with the emotional effects of DMD and their inability to keep up with their peers. To then throw AFO's into the equation can overwhelm them - especially if they are experiencing sensory issues, or have other neurological "challenges"
Their gait also changes as the contractures begin and their heels lift off the floor, so to introduce AFO's at this point (they are quite hard to walk in) complicates the issue further. For our son he felt so unsteady on his feet (and more so when wearing AFO's) he would not move once they were strapped on, and would ask for them to be taken off if he needed to get up and go to the toilet.
I often watched the other boys "charging" around in their AFO's on camp as if they weren't there, and found myself wishing we could have had the opportunity to introduce them earlier when he still had the strength and flexibility in his ankles to learn to ambulate in them without fear of falling.......
From a parents perspective (and sometimes we forget that we are affected by each new therapy or decision) I would have loved the time to source socks/stockings, gripper soles and adhesives without the pressure of needing them NOW. To have the time to try things out, and if they didn't work not feel stressed (because the AFO's were not being worn that night) would have made the whole process a lot easier for the entire family.
Obviously it is a personal choice, and it seems you are quite happy with your Dr and their decision, and your son has taken to PT like a duck to water. However you asked for the parents feedback and we are providing it, so please don't think that we are all saying you MUST use AFO's - we are just giving you feedback from our experiences. :)
Just an update. Our PT recommended the night splints. Josh has fallen arches and his ankle alignment is a little wonky so his heel stretch looks a little better than it actually is. So, we'll be getting Dafo 9 Softys at a cost of $600 but our insurance plan should cover it.
Josh is actually looking forward to it. He keeps asking when does he get his stretch boots.