Dr Ray There isn't a student in existence, and precious few instructors, who has studied the various different schools' manuals in order to make an objective assessment of their relative merits, so you have absolutely no chance of getting a reliable answer to your question.
Of the publishers you mention, all provide excellent information for the examination system they are designed to service. Pooley's sell at least three of the four you mention, so why don't you ask them what they think? They're bound to have a few instructors in their staff who've compared the books.
That said, the point made above by several of your respondants is valid: why buy them at all? The course your son has contracted for will include all the relevant manuals, and I'm sure BGS will provide them in advance if requested to do so. It seems to me that you're hoping that spending extra money now will help your son's chances in the examinations later. If you want to do that, get him some private tuition. I'm sure it's available to those with enough cash, and I would think that whatever study materials are required could be provided. However, I think you'll find that the BGS course and a fair bit of work on your son's part will be quite sufficient to pass the exams.
As your son is still studying at Uni, don't you think he deserves a bit of a break before he takes on the huge workload of the ATPLs? Spend the money on a holiday for him!
Scroggs