Hi David,
I wouldn't worry about him not using the crutches as long as he is walking without a limp. The fibula is a non-weight bearing bone and so thats why it isn't in plaster for as long.
Your rehab programme sounds about right. Is the support he is using a the moment removable? If so i would start gentle mobilisations at this stage. As you say balance is the big thing to try and reduce further problems.
With the strength aspect, if the patient is tolerable and no pain start isometric exercises asap, then do it partial WB and move onto fully WB. When bilat heel raise, try and ensure they are not just using their good leg, but unilateral will find out the cheats.
Let us know how you got on.