Just a thought, shouldn't he be charging your son V.A.T. at 20%?
So, of a £100 job, your son will make £51.50 where his mate will make £55.75
That's what I worked out would be the issue - re the disparity of net to each you can slightly weight the split so that they both end up with almost the same net after paying the VAT (don't need to get too anal with decimal points & pennies).
As far as I can see the best you can do is minimise the damage & as said it is only going to be a temporary issue. Think that if they can agree then in would make sense for Andy to invoice my son monthly for half the work done (weighted to allow for the above) net of his share of the workshop expenses as this will reduce the overcalculation
Welcome to 2018...
Think you have to go to your Amazon account and delete.
As there are in your account your Kindle will pop them back on whenever it syncs