You wanted numbers.... So looking at the bills you have stated you have bills of
£200 council tax( you shouldn't be claiming reduction with him or his adult son living with you),
£70 internet (how? That can be reduced!)
£80 water and
I'm going to suggest £400 gas and elec. For price and use rises
So £750 a month? Plus £1500 rent currently? £2250 total
By him moving in you will also loose the universal credit as his income will be counted (as will his adult sons I believe...)
Either that is split by person
You, 50% of 2 teens, part of 2 younger kids =3
Him, adult son, small part of 2 younger kids =2.5?
Don't know whether that's a fair add up - but it will do!
£2250 / 5.5 = £410
So £1227 for you and £1022
If you don't count his little kids and reduce yours
Him =2, you =2.5 it makes the maths easier £500 per person so £1000 him and son and £1250 you.
Even including your smalls as a whole person the split should be 2/5ths 3/5ths so £450 per person or £900 him and £1350 you.
Either way, pre move you and your kids are subsidising him by around £1000 a month. Before you add in food.
If you look at post move with mortgage payment your total house spend is £1630, using the 5ths split this would give you £326 per person, £652 him and £978 you.
I don't personally think you should not charge him any kind of rent contribution - I k ow others have views on this because of the legal position of claiming ownership.
If you go with just the £750 running costs then £150 per person - £300 him, £450 you.
But bear in mind that none of my calcualtions include any food/groceries etc.