I've been in a situation very similar with my DH the past 9 months. He never used to earn very much, and was always on short contracts which he would struggle to find work after. The past 2 years his salary has been x5 what he made in a "good" year before.
We've gone from on the absolute bones of our arses to having a life, some stability etc.
The problem is he is an outside contractor, and not seen as part of his team at work, he's suffered a lot of alienation at work, and then spent the nights in a hotel on his own, at this point depression started to creep in. He's been really very unhappy.
We had to make some changes, he now travels to and from work each day, which is a 4hr commute, but it does mean he gets to enjoy dinner at home, and a cuddle with DD and has the opportunity to drink a cup of tea and let off steam about how he's often treated at work (verbal abuse is sadly not irregular. When his job upsets people he works with, as a group they'll generally start with name calling, and tell workers that he's unhappy with something they've done, resulting in tools being thrown at him)
Its a shit situation and he's wanted to walk away from that contract- and I have supported him in that decision, only for him to say that he needs to stay because if he walks away, when the client is an industry leader, he risks his chances of ever gaining work with them again.
My advice to you is simply save as much of his wage as you can, support him. See if there are any contracts more suited to him, if there are, help guide him in what training will maybe help him become the first person people will look to for the contract he will enjoy.
DH is hopefully starting another contract in two weeks time, he's leaving the current one early for another contract with the client, and I am hoping that it makes him happier, because at the moment he is unhappy, and also really hard to be around.
Also, as PP have said, therapy. He sounds like it would be very helpful to him