I am very sorry that your son did not make it to the interview stages at the 2 schools, sluggishMum. The whole 7+/8+ process is so gruesome, and only getting more competitive each year.
The first thing I would do is call and ask the schools for feedback on your son's performance. They will be candid and go into detail as to what areas he did well, and not so well in. I would also ask if they thought that he would be a candidate for 8+, or that he would need additional years of development and thus 11+ would be a better option. The three schools you had your son sit for are very tough to get into, and even tougher to thrive in if he is not able to handle the fast pace of learning.
There is no magic formula for getting in - for english, have your son read widely and check his comprehension. Bond books are fine for comprehension, but target the year above his age range. Maths, wok on mental maths and speed. Also, make sure you cover the full curriculum for year 3 prior to the January exam - his prep school counterparts are doing just that. Prep schools seem to use the Maths on Target series. My son, who is at one of the schools, is doing the book for the year ahead of his current academic year. Verbal and non-verbal reasoning - bond is ok, but there is another series which is better (I'll get back to you on this as I can't seem to remember the name). Practice story writing also. Colet's exam is heavily focused on maths and reasoning (makes sense as St. Paul's particular strength is in these areas). King's intake at 8+ is much smaller than 7+ - usually 5-8 spots only, however it does take a wider range of boys. I am not familiar with the Westminster exam, but I have heard that it too places the emphasis on maths and reasoning.
Just a warning, the level expected at 8+ is much higher than 7+ - much more than just a years worth of development, so if your son really struggled at 7+, perhaps waiting for a different entry point might be wise. However, if the schools say that he in general did well, just not well enough on some sections, definitely go for 8+. 11+ is a better entry point as it is geared towards state-school candidates. Colet Court even does a deferred 10+ entry for state school boys, so your son would take the exam a year and a half prior to entry into Colet.