Hi Gilby
Even with CashPlus from a local store you won't get the card that is suitable for use with a hire car by the weekend...the cards that you get from the merchants are not properly embossed as they are only temporary cards which work in ATMs. As it is your son will still need to send off by snail-mail copies of documents to CashPlus to get the card verified and suitable for use with car hire. You might try phoning your own credit card company and see if you can get your son added as an additional card holder on your account...and see if they can Fed-Ex a card out by the weekend. This is probably only likely if you are a Platinum AmEx card customer or equivalent.
I was in the same situation a couple of years back when I had a car booked to pick up in the USA. I read somewhere (or heard from someone at Avis like you did) that a debit card would be ok. I phoned the rental location to verify and was told that this is only ok if you have an existing relationship with Avis and a rental history with them and one of their privilege cards. Luckily I had a few weeks notice so my travelling companion was able to get me signed up as an additional card holder.
Probably just best to invest the time, energy and resources in taxis and public transport...or see if there is a car hire facility at a local garage there that will take a debit card or cash. When your son returns, perhaps he can look to set up a new relationship with another bank that moves a bit quicker than his current one! Good luck!