£100 a month is a joke, so I'd definitely go through CMS!
Child maintenance will likely price him higher than that, of course given his financial situation. One thing I learnt from my mum was to never have a casual agreement over child maintenance, they always find a way to not pay because they know they can, can't do that when it goes through the system.
Definitely go through child maintenance as If his working it would Definitely be higher than that and he knows this, which is why his messing you around.
go through child maintenance online and they will say how much he has to pay