Good advice above. It needs to be his choice - you can't MAKE him work and even if you could, you can't make him try in the exam, as parents are banned! Plus a stressed child won't perform to their best level anyway.
So talk to him about which school he'd like to go to, and if he doesn't know, discuss the pros and cons, and visit if not done. If he does want to go to the grammar, and just needs motivation to do well, then I second bribery esp for success in spotting careless mistakes (not fair to penalise someone for simply not understanding the topic or not having been taught it). If he actively doesn't want to go to the grammar, you can try persuading him, but ultimately it needs to be his (informed) choice, or nothing you will do will succeed and your ds will hate you for it.
Re stress, I told my dd (who did want to get in) that all she could do was her best on the day and that the examiners didn't have a little camera to see inside her brain with; all they could measure was how she performed on one day in one exam - which wasn't necessarily a reflection of her real ability. So if she passed - great; but if she failed, it didn't 'say' anything about her actual overall ability in other, future exams or in life. No big deal.
And that she could and would do well whichever school she went to. In our case, I actually preferred her second choice, a comp - so could in all honesty make it clear I wasn't bothered whether she passed the 11+ or not - I really did let her make her own decision on this. As after all, she would be the one who goes there. It meant that when she slacked, I could remind her that she was working to achieve her own goal.
I'd also start by doing a practice paper in each subject to be sat. The results may be poor - that's fine as it allows you to do another one in a few weeks or months (or even the same one) so he can see clearly how much he's improved - very motivating. It also highlights what topics or areas he needs to work on.
I also wouldn't do it every day = overkill. Everyone needs a break sometimes. If you want to do 3 hrs a week, better to do 3 days of 1 hr or 2 of 1.5 hrs and the rest off.