Ok - thanks for the extra details.

Try practising kicking a ball as this mimics swing phase and you can easily break it down. Start in a high sitting position (the higher the better but dependent on ability - he needs to achieve active knee extension) as this will fix the hip and he can concentrate on knee extension. As active extension gets easier - get the position higher and the support less i.e. gradually working up into full standing.
You could also try some PNF both for knee extension & flexion
Squats without UL support and actively using hip & knee flexors to pull DOWN into the squat may also help (you may need to provide resistance in lying to give the patient a sensation of this - normally we just let gravity take us down.)

Also practise marching- if he is circumducting when walking he's going to get a pattern of shortening in the trunk & daily practising of marching (as you need no push-off & little knee flexion) will give him time to left his pelvis drop on that side. It's also good to practise arm swing at the same time which helps with appropriate trunk movement (or lets you see if there isn't) and can challenge balance/co-ordination. If he can't let the hip drop then work on it as this may be adding to the effect of the quads & hamstrings in inhibiting knee activity.
Are you practising push-off in active stance and assisting gastroc contraction?

It seems that the patient has overactivity of quads & hams which could be to decreased trunk stability (?) or as compensation for inactivity further down the chain. You could try taping to inhibit the action of both while walking but this is unlikely to help if gluts & abs are not providing sufficient stability.

Good luck and keep us updated!