If you realistically calculated it all you could add it all up and see that uniform is an extra.
Difference between current rent/mortgage and average cost of a place with X number fewer bedrooms/no garden/not near schools (or whatever considerations you jointly or independently made when choosing current house with children in mind). If he has done this too perhaps disregard.
Estimated proportion of weekly/monthly grocery cost, electricity, water, other utilities, TV, internet.
Estimated cost of ordinary clothing (including shoes) per year/half year/month.
Cost of any subscriptions, memberships, extra classes.
School costs - uniform, stationery, bags, sports kit, transport, trips, school dinners, PTA fundraisrers.
Childcare/after school club costs.
Estimated proportion of car costs/cost of child's public transport tickets
Cost of large one off purchases e.g. bed, car seat, safety gates, musical instrument, sports equipment, birthday parties. Unless equivalent bought for own home.
Less child benefit or any child tax credits etc.
Might be depressing to calculate this way but would at least give you an idea of realistic/fair costs.