Not Rtft but have read all your updates. Your son sounds identical to mine so completely understand where you’re coming from.
It’s fairly common for grandparents to fund all or part fees in the private system; they should have worked out what they could afford before they offered. However you don’t want to upset them further and lose their contribution altogether. Could you have a conversation about them possibly helping you out with uniform costs, or the ongoing sports kit costs? It was £6-700 for full uniform for mine, plus top ups for seasonal changes, then £100 for sports equipment like hockey sticks, tennis racquets etc. Encourage him away from cricket as that is very expensive for the full kit!
All is not lost. You must ask for a bursary. It’s tricky as the Y7 bursaries will have been given out now, but there may be a pot for in year hardship funds. Sounds like your son would have done well, so he is the sort of child they want. I have 100% bursaries for one of my children and a 50% offer for another so I am speaking from experience. It is super awkward asking, but the bursar is totally used to it and will be understanding. If their pot has gone for this year, you can apply next year and stand a better chance of success if your son is doing well - they would rather keep your child then use the pot for a new child if you see what I mean. They may offer as if about how to deal with the GP contribution, and point you towards charities.
Other options: 1. ask the school if you can pay the full 5 years now. Some allow you to do this and you avoid the year on year increase. You take it out of your mortgage and use the money you would be paying back and the GP money to pay the increased mortgage. This will mean that as time goes on and your husbands salary goes up, you can start overpaying the mortgage. I wouldn’t recommend this as you are tied into a school which may not actually work out for him. But it is an option to consider.
2.don’t send him on any trips, use that earmarked money for the fees. It is normal for bursary children not to go on trips. Seems like you have ear marked 3k for this? He won’t be the only one who doesn’t go. Depending on his level of need, he may not even want to.
3 make cuts everywhere else in your lifestyle. Holidays to the wire. If it really is a choice of home school or this school, it will be worth the sacrifice. We have one annual holiday camping. We know exactly when we’ll have money for holidays again and the kids are aware we have different lifestyles from the peers. Some of their pals are mega loaded; they learn to choose friends who don’t just care about money, which is a skill for life.
4 there are companies that will lend you school fees, with very long repayment terms to make your repayments more manageable. This is expensive because they are paid over a long time, so you end up paying a lot in interest, similar to how mortgages work.
5 for us, our eldest was at the school when our contribution doubled due to large fee increase and then VAT. We dealt with this on 0% credit cards until our salaries increased. Absolutely NOT recommended; we can’t wait for the 50% kid to leave and we can get everything repaid (we got ourselves promoted and can now cope with the fee increases, but no extra to repay the debt we accrued whilst we couldn’t). But, there was no way we would remove them after they had started - they love their school and the local state is terrible, and with their AuDHD they would have got on really badly. They have struggled with autism and have had huge amounts of time off. They now have part time provision but still on track to get 9 GCSEs @ grades 7 and 8 (was originally on track for 11 x 8s and 9s). If we had gone with the EHCP offer the max they could take was 7 GCSEs, and the cohort usually get 4s and 5s.
Even tho my children’s school, as an academically intensive private, isn’t actually very good with neurodivergence understanding, (we are the only EHCP) neither is our state school provision and so it is still our best option. We very much feel that we are paying for our child’s future, and if it cost us our house it would be worth it.
FINALLY - if you do end up sending him to the private school, make sure you get insurance which refunds you in the event of him being sick and missing school. My child missed two full terms for sickness - which was autism related. We had to pay our full contribution which we didn’t quibble as we didn’t want to lose the bursary we did have, or impact our other child on 100% bursary. Ultimately your son may do fantastically, but if he doesn’t and you are stretched, it will feel that much more painful.
Best of luck whatever you do