I think that I would start with a realistic assessment of your son's chances. A super selective like Judd is aiming to take the top 5% of the school population- so in an average class of 30 probably one or two children (and that would be if they were all boys!). Obviously this will vary a little by area so in a more affluent area with high parental investment or a private school there will be more in one class who would qualify. You could get him to take a standardised test looking at English, Maths, Verbal and Non Verbal reasoning. He would need scores averaging over about 130. He also would need to pass all of them so no really low scores. A less selective grammar you are probably looking for scores in the region of 115-120.
This could be your lucky year as many companies who would usually do physical mock exams have had to go online using zoom, likewise many tutors are also doing online lessons. It is worth seeking out a mock test from a local company for a realistic assessment of his chances.
You also need to look at the date requirements for applications. For Kent you need to apply in June to take the test. The test itself is early Sept. Results out mid October. Applications in by end October. You need to check for residency and also if flying in for the test consider and quarantine regulations in force at the time.
It will be tough as some children will have been at private schools AND been specifically preparing for this test since yr5 or earlier. It can be done but may be difficult even before you consider the difference in syllabus, for example maths includes algebra and ratio which even in UK isn't covered until yr6.
Another option might be, considering that he has come from a different system and depending on when his birthday is, asking for him to be kept back a year so go into yr4 and then sit the test next September. It is age adjusted so he would need to get an even higher score, but it would buy you time to settle.
Eleveplusexams.co.uk has a forum with very knowledgeable parents who will be able to give you specific guidance for the schools you are interested in.