So guess you need to decide if the school won’t budge on household income whether you split as a couple or don’t live together for the next 3/4years so they the boy can stay at school via the bursary
if dp moved out would he get help via uc and rent (depends if the house is Co owned)
how long have you been together /live together @CloverRiver
is the mortgage joint or did he move in with you (as you seem to have the money)
and I get why you don’t want to spend £43k over the years as it’s less in your pension pot or to inheritance to your older kids