As advised, depression is something to be assimilated, as it may be a long term, even lifelong condition.
One of the first things the patient, and family, need to do it to accept the condition correctly - it is an illness, and will respond to medication.
Depression responds to antidepressants in the same way that diabetes responds to insulin - it's important for everyone involved - especially the patient - to adopt that mindset.
Another vital factor is to remind the patient at regular intervals, that he will get his personality back. It has not disappeared, it is a little out of focus, like binoculars, and the correct adjustment will bring it back.
Medication takes at least three weeks to settle, so working out if the change is a correct one may take some time. It really is trail and error because medication that perfectly suits one can have horrible effects on another, so tell him to hang on while things settle down.
As far as exercise and diet, his healthy regime will certainly help, but on the days when he is lethargic, or feels like junk food, accept it, and go with it - no-one is waiting with a gold medal because he eats his five-a-day every single day.
The way to deal with depression is to enjoy the average and good days, and ride out the bad days. It's not like a broken bone, you don't make continual forward progress from accident to healed. The process can feel like one step forward, one hundred steps back, but it's not a compeition, no-one is keeping score, make sure you don't either.
Your partner will have days when he utterly hates himself, and will feel totally unowrthy of your love and affection. On these days, he will put quite a lof of effort into making you hate him as much as he hates himself, which will make him nasty, spiteful, morose, withdrawn, and generally difficult to live with.
Ride it out - he does not mean the things he says, it is his illness, and it will pass.
With love and support of you and his family, he will get past this horrible time, and learn to pick up his life again.
Nietzsche said that what does not kill us makes us strong - this is what he was talking about.