Hi there,
If your partner is getting pain, it's not in his head! Do you feel that the hospital are trying to put his pain down to psychological problems? It doesn't sound like they are doing much to help you!
St Johns Wort is very useful for mild depression but it won't help with physical pain. If you want to go for it, you need to buy a reputable brand, and follow the instructions. Also, bear in mind that the results won't be instant. He may not start to feel better for 3 or 4 weeks. It takes a while to 'kick in', so he should be aware that there is a time lag.
I am assuming his knee is not infected anymore, and wounds are all healed etc, and there is no inflammation or swelling?
If that is the case, the pain he's getting might be neuropathic - that means that his nerves are stuck in a cycle of sending pain signals to his brain. This is probably not surprising considering all his knee has been through!
There is a drug which is very helpful for this sort of pain. It is called Amytriptyline. Taking this drug is very useful for breaking the cycle of pain, and it is also quite safe because only a low dose is required. Your GP can prescribe this. The irony is that in higher doses, amytriptyline is also used as an antidepressant! So you can't take it at the same time as St Johns Wort.
If your partner genuinely feels he is developing depression, then, yes, St Johns is a great natural alternative to conventional antidepressants. But it sounds like he'll feel better once he gets his leg pain under control. It's a bit of a chicken and egg situation. If eliminating his pain would eliminate his depression, tackle that first.
Best of luck !