I presume you mean The John Lyon School in Harrow.
My son is in the Lower 6th at The John Lyon, having been there since Year 7 .
The entrance exam included an online CEM type assessment and a written English essay ,followed by an interview ; although this may have changed over the years.
My son has thrived there walking away with amazing results at GCSE If your son is motivated he will find all the encouragement he needs . The boys come from a wide social mix and I know that some bursaries are very generous .I have no idea of the sliding scale but I do know that a combination of a scholarship and a bursary mean next to no fees.
It is rather undersold as a school but has everything you woukd expect of an independent school; great sports,music and enrichmnent activites ( DOE and CCF) .