Well with Ds really keen to stay where he is, personally I don't think it would be too odd to let him know the situation -ie We'd like to keeep you there too, but need the school to contribute to the fees, to do that you need to excel - are you willing to put in the time and effort needed to shine, even if it means less free-time for yourself, for a year ? -0 see what your lad says, some children of course will not want a commitment, which is fine in your situation as probably a transfer to gram would still be a good education and wouldn't shut any future doors for him, for mine the secondary options here are awful.
Also you could always give pushing him hard a go for a couple of months and then re-evaluate -0 he might well surprise you, mine grumped for the first week of 2hr daily additinal tutoring (by me) but after three weeks was so used to it he felt he was doing less than his friends,(not the case at all_) children tend to be OK with a situation if they feel everyone is doing the same, that they aren't missing out -0 this illusion can be created by wily parents intent on keeping good family relationships whilst at the same time optimising education.
If you are considering a deliberately taught path you will not be popular either here or in RL - in the East you'd be applauded !
The thought that children should not be forced to work on something they may not like or be "naturally" good at, stems from anxious thoughts about childrens self-esteem - my view is that there is no greater way to enhance a child self-esteem than for them to work hard, rise to a challenge, and be successful.