I don't think you'll get anything if he's claiming UC because I didn't when my ex was claiming. Maintenance doesn't effect how much UC you get so they won't make up the loss
Go on to the Turn to us Benefit calculator and put in all of your new details. It will tell you what you will get each month. You could fill it in a couple of times so you could see what the different payments would be with the different amounts to put in
If he goes on benefits you’ll get a small amount from him !! But you won’t get extra from child maintenance