Forgive me for being so blunt, but you clearly don't understand much about depression.
Depression can take many, many forms. It is not, as is the popular opinion of even many GPs these days, a simple case of thinking your life sucks and you just can't be arsed with anything anymore.
When I had a breakdown at the age of 25 I spent 11 months as an outpatient in a psychiatric ward. I had to take the time off sick from my job, and was eventually made redundant, because there was no way I could've continued to work throughout this time. Not because I 'didn't feel up to working,' but because certain very specific aspects of my job would cause me to have severe panic attacks, which of course would have been far too disruptive for my co-workers to deal with on a day-to-day basis. Other than that I could function perfectly normally for most of the time - I still went home at the end of every day and cooked, did all the usual household stuff, etc. I certainly didn't walk around 'being depressed' all the time.
Incapacity Benefit is damned hard to get for mental health-related problems - I had to go through all sorts of hoops to get mine, and they whipped it right away from me again practically the minute I was discharged from the psychiatric ward. And if you really want to know how hard it is to get back into another job after a long period of depression.. well, try telling a potential employer at your next job interview that the recent gap in your CV is due to you having treatment for depression! Doesn't exactly shoot you to the top of the 'yes' pile, no matter how well-qualified you are for the job. I was out of work for a further year after being discharged from hospital, simply because no employer would touch me with a bargepole for that long.
His parents gave him the holiday because they obviously thought he needed it - and who knows, perhaps he did. How should it have gone then - should he have spent every minute hating it and crying at the futility of life until he got home, in order to 'prove' to you that he's genuinely depressed?
If paying maintenance for you and your kids is your issue, then that's your issue, not his mental health. Therefore I think it's unfair of you to try and strap it to his depression and claim that it's just his excuse for not getting a job that pays enough money to hand over to you. And if you're hassling him about it all the time you aint exactly helping him recover enough to GET another job.