I went to Hendersons for the JMSS test. From my time at Hendersons, it felt quite rushed and personally didn't help me that much. Hendersons mostly rushes through topics; for example, they put all the maths topics in 4 days and 1 days to teach us all the science content which made the content felt very rushed and it made it difficult for the teacher to explain in detail. However, because of Hendersons my writing improve greatly as there was people checking my work and Hendersons also provides you with good resources for practice.
My opinion based on my time at Hendersons is, go if you need resources and your son is already advanced in class and just needs a refresher or quick run through the topics. If not, Hendersons is quite expensive for a couple lessons and tutoring centers like JAC is better in my opinion.