Wizard - this is my personal opinion and I'm sure many would disagree with me, but I don't think you should give the speed thing a second thought. With 2 weeks to go, you risk making him anxious that he isn't going to finish the papers.
One of my sons did the 11+ last year and he was very anxious about the whole thing, but got very agitated if I tried to time him. He had a pink fit if he so much as saw me looking at the kitchen clock when he was doing practice. He was taking twice as long to do NVR questions as he would have in the real tests but any attempts by me to get him to speed up would just increase his anxiety levels and I had to take an extremely gentle approach with him. But I worked on the assumption that, with practice, he would start to speed up naturally and that it was better for him to focus on the questions, work through them efficiently and not be distracted by how much time he had left. I think he may have guessed a few on the VR paper (but we'd discussed the fact that lots and lots of kids wouldn't finish that paper as it's so time-pressured and he mustn't panic if he didn't, just quickly fill in any remaining answer boxes).
You may also find, if his maths scores are dropping, he's getting bored. He's not likely to be getting worse at maths so he's probably just fed up with the whole thing. My eldest regularly scored around 60% in practice papers with me, which he would rattle off in 20 minutes. I felt like tearing my hair out, but he just used to say he'd put the effort in for the real thing, and he did and passed comfortably.
With two weeks to go, all I've ever done is maybe half a page of VR every few days and laid off completely a few days before the tests started. What I would advocate for maths at this stage, rather than practice is taking just a few minutes to run through a few key things with him. For example, you might check his terminology (can he remember the meaning of product, mean, median, mode, quotient), check that he knows the different types of triangle and what their properties are, how to work out areas of rectangles, squares etc.
Good luck and hope all goes well!