@mickle has shared his cleaning routine. Think the 'salt in washing up liquid' thing is massively overdone, since you'll be rinsing off: the residual concentration is minute.
But since the OP comes here asking for advice, I think you need to be working on sonny Jim, on two points.
* Have you influenced this 'brand recognition means it must be good' syndrome your son has caught? Whether you have or not, I'd counsel trying to influence him to be rational and think for himself as opposed to gobbling what advertisers offer and 'wanting' only that product range. Who's paying? In what way does muc-off "give off an image" please?
* Bit concerned that you may be washing his bike for him. You will do him a service if you can encourage him (aged 13) to do this himself, and make self-sufficiency a life habit. Who cleans his football boots? Who oils his chain?
[Yes, I have a teenage son!]