Standing and walking promotes bone growth but your son must consider the full weight bearing limit of the LL device.
Stretching is of vital importance too in order to avoid muscle and nerve shortening.
Other activities like stationary cycling and partial weight bearing while walking in a swimming pool would be beneficial only if your son is comfortable getting into a pool with the device.
Multivitamins, glutamine, and calcium supplements may help with bone and muscle recovery. Say no to smoking and there are some speculation that NSAID painkillers may inhibit bone formation.
During consolidation, your son should not forget stretches. I had plenty of pain on my left leg during the 6th month of consolidation because I gave up stretching and focused more on walking and muscle mass. However, the pain resolved once I resumed stretching and now I walk fine without any pain.
Anyway, make sure that your son gets plenty of sleep too.