I'm afraid I don't have any personal recommendations, but as nobody else has replied I thought I would!
My DD did the 11 plus in Birmingham several years ago, and although lots of children were tutored, it isn't strictly necessary, so it may be worth thinking about a DIY approach, particularly if you are planning to start in Y4.
The main thing I did with DD in Y4 was lots of reading aloud together (taking it in turns) and discussing the meaning of words/phrases, working on inference skills etc. This felt a more meaningful way to build vocabulary and accurate reading/analysis skills.
In Y5 she did workbooks (CGP were the favourite but others as well) for familiarisation and fluency. In maths I did teach her long division and ratio/percentages but I'm not sure that was necessary. Mostly it was just working on fluency in workbooks and starting to do timed work from the middle of Y5.
The most helpful thing was going through wrong answers carefully. Also spending some time looking at a new maths question and discussing the different ways to approach it and working out which was fastest - helping her to make the best choices on the day. A lot of it was about being able to understand the question quickly, and this is why lots of 'intelligent' reading in Y4 was so helpful.
Assuming your DS is already working ahead of expectations at school, there is no need to do formal tutoring as the actual difficulty doesn't exceed Y5 curriculum standards. It's all about grasping the question quickly and working at speed. Of course, familiarity with the types of questions is very helpful, but that's easily achieved with workbooks.
Another helpful thing was a mock test in the summer of Y5. If you choose a provider with quite a big cohort for the tests, you will get useful data on how your DS performed compared with the other children sitting the test that day. Of course, it's also useful exposure for your DS to the atmosphere of the 11 plus. But I would suggest only doing one or two mocks.
I think I was lucky because DD's primary school was not in a particularly affluent part of the region and tutoring wasn't rife, so I didn't feel any pressure from other parents to keep up. We just did our own thing and she passed very easily and is thriving in her chosen grammar.
Of course, if you do go down the tutoring route - fair enough and good luck.