Aaaah poor little soul 
Good signs is he's eating (anything like shock is bad for guinea-pigs and they 'close down' )
The weeing and poohing is good - poohs have to have muscle control to get them out, wees not so much, they could dribble uncontrollably.But if he's poohing then it shows he can still put some effort in.
You might need to tempt him with his favourite foods and even spoon feed him water if he can't do it himself.
The feeling in his back legs is a good sign too.(Sensory nerves)
The lameness might be that his spine is bruised rather than permenantly damaged.
Once the inflammation goes down you'll have a better idea.
I'm assuming the vet gave your boy an anti-inflammatory and painkiller - by injection ?
Have you got any medications from the vet to give him?
Keep him quiet and calm. He's in a vunerable state right now. Shock can literally see them off so be prepared.
No flaming from me- last year my GP1 (and I've kept GPs for years) leapt off my shoulder onto the wooden floor of the Pighouse. He 'just' missed bouncing off the lid of the haybox.
He was (understandably) shocked, as was i. DH and I moved all his joints carefully.He hadn't lost any use or feeling.No blood or broken teeth. But he was a bit 'whistly' for a couple of hours.
I put his soft pet bed in his cage, put his brother next door (seperate) to keep him company and agonised if a trip to the vet would traumatise him more.
He started eating ,breathing properly and quietly within 2-3 hours. We kept him in a small area for a couple of days, he was a bit sorry for himself but was walking well.
One life gone.I can still visualise those little legs as he landed 
Your pig might not make a full 100% recovery. I guess you'll need to do the Wait and See Approach and take it from there.
Hope he's made huge improvements by your next vet visit.