30 months is quite a long time in terms of recovery. He may have some limited potential if strengthening exercises haven't already been applied. However I think further improvement is likely be limited. A trial of intensive strengthening would tell you. Using FES in combinatin with volitional effort for the weaker muscles - particularly the plantar flexors might be worth trying. However even if you make some strength gain it may not be big enough to translate to a functional gain.

However at this stage I would be thinking of making therapy very functionally oriented with task oriented training. Given what you describe he should be able to manage some level of independent standing already at least of a relatively high surface. He should be managing some sort of walking at least with an aide as you say most muscles in the are of normal strength except for the hip extensors and plantar flexors. Is he? If not perhaps you need to go back and reassess his lower limb strength in more detail