Packsafe Coversafe X100 RFID Waist Walle
I was checking that Pacsafe Coversafe x100 waist wallet out and came across some secret pouches that look very slim and are worn under clothing (some worn on the wasit, others on the neck.)
Also, this Pacsafe wallet that attaches to your belt loop looks good.
https://www.amazon.com/gp/product/B01M2DJORJ/ref=ox_sc_saved_title_1?smid=A1F1CRO6ZMXV7T&th=1
I think with his sling bag, he would want somelthing lighter/more hidden for his valuables.