Ex won’t pay me what I’m entitled to

Myself & my ex have been split up for almost a year he works a full time job & works mega overtime. He takes his son 2 overnights a fortnight. He gives me money but with absolute great difficulty. I’d like to have it done properly as I’m a student & need support & the proper amount I’m entitled too. I think having it done properly would make all the difference. He is on £10 an hour & works 40+ hours & does alt of overtime. I’m not sure if that’s on the side though.