The fact that he has SN does not affect your right to home educate and (assuming you are in England and he is not at a special school) you only have to de-register him from school and you are then free to start HE.
If you were in Scotland, or he was at a special school, you would have to request consent to withdraw him from school (although they can't unreasonably withhold consent, it can still take them a while).
The only other difference is that the education you set up for him has to be suitable "to any special educational needs he may have" as well as his "age, ability and aptitude".
As to teaching him for 6 hours a day, don't worry about it. He might be learning for 6 hours a day, just as he did before he started school, but that doesn't mean you or DH have to be actively teaching for all that time. Huge amounts of learning happen when you are just doing normal things together.
Even if you were going a more academic route, that can be done with far less direct teaching time out of the school environment. My son attends an internet school doing 7 GCSEs, and he only has 3 hours a day of teaching time. Many HErs with younger children reckon about an hour a day is enough to cover the school curriculum (if you wanted to).
There is no reason why he would need to go back to secondary school, unless that's what you and he want at that stage. It might well be that a college course in a practical subject or an apprenticeship might be more his thing. Either way it sounds as though the school is not the best environment for him at the moment, and HE for a while at least can only help!