Husband with tinnitus and I'm getting so worried.

I'm here asking for some serious advice. I'm getting more and more worried as each day goes by and I have no-one to ask, so I've turned to this forum in the hope someone can give me some positive help.
Just over a year ago, we came back from a holiday abroad and my husband noticed that he couldn't shake off a high pitched tone in both ears which seemed to have developed after getting off the plane. A couple of months later he still had it so went to the doctors who found nothing obvious. He has Private Health cover through his work, so he kicked that into action.
Anyway after hearing tests, all resulting with no hearing loss being shown except for a 10% age related dip around 6K, an MRI, much prodding and poking, confirmation came back that he had tinnitus, so go away for six months and try to live with it. Habituation will happen.
Well as you can imagine, it didn't and I've watched my husband start to go downhill. He says it's there 24hrs and he's aware of it 100% of the time. It repeats the same thing every single day.
He wakes up with a level 4 or 5 out of 10. He describes it as 5 is treading water with this mouth above the water line. Any higher than a 5 and he's starting to drown.
He stays at a 4 or 5 most of the morning but from about 3pm it starts to rise to a 6.
Around 7pm early evening, he's either a 7, 8 or 9. If he ever got to a 10 he says he will commit suicide.
Fortunately, he's never got to a 10 as somehow miraculously, he always falls to sleep at around 10.30. He does have a problem staying asleep but nightly amitriptyline helps this.
During the next few months we went to our local hearing centre to try a generic hearing aid with a tinnitus masker. He only used them for a couple of weeks but could tell they were not helping.
He then went back to the doctor saying his mood was low, daily headaches, crying most evenings but I'm getting more and more worried about how close is he to becoming suicidal. He says it feels like being on a cliff top in thick fog and doesn't know where the edge is.
He may be safe and the cliff edge a 100 feet away, but then again it could also be only 1 step away. That's the scary bit. Would one more thing force him to take the final step?
The doctor prescribed 40mg fluoxetine each morning and 20mg amitriptyline at night to help him stay asleep.
He's now also paid privately with a tinnitus clinic and wears specially tuned "noisers" hearing aids about 8 hours every day from 1pm to 9pm.
We know his tinnitus is reactive to noise. He's been a computer service engineer for over 30 years and drives long distances to various data centres, (noisy) or offices.
Since they were introduced a few years ago he's always worn Hearing Plugs or Ear Defenders but they have no effect as the same daily pattern always repeats.
He comes home from work and then can't wait for sleep which obviously then gives him some relief. This is no way to live a life.
The worrying part for me is that over the last couple of months the levels 7, 8 or 9 are becoming earlier in the evening and now seems to bother him from late afternoon.
I'm worried that as each day goes by, his job is making him worse and if he continues, within a couple of years he'll be on a high level as soon as he wakes.
I know he has PHI income protection through his work which pays about 75% of his gross income until either he gets better or retires. He's 53 now so has 14 years left.
I've asked him if he can think about using this but he thinks this will only pay out if the illness stops him from working and not just because it's making him ill.
He explains if her were to lose both arms and couldn't drive a car, they would pay up.
But if you get severe tinnitus because you are working, they would not. He also says working passes the time, but I can see it's making him worse. However, I have noticed when we have holidays, once the first week has passed, he is more like his old self, only getting the level 7,8 or 9 from 7pm. With the help of the noisers, which give him a two hour relief, we are able to have a relatively normal life again.
Does anyone know if these company PHI income protection schemes do pay out if work is causing the tinnitus but just as important, how can I prove it?
I'm watching my husband spiral and I want to know if anybody has actually been able to use anything like this.