An AFO may be worth exploring if it will give him better functionality and less pain as a short term. It may decrease whatever aggravation is being caused in the ankle. BUT, I think you nailed it with the words transitional and muscle development. My orthopedist considered one for me and PM&R immediately said no for the reasons of impeding muscle use and ROM, hence setting up future injury and imbalances. It will take extra work to maintain his calf muscles.

If it makes the difference in weightbearing for now to get to appointments instead of either being in too much pain or using a higher level of assistance then that could be helpful.