Your son got screwed. My step son had this happen to him in a Euro tour - he was quoted a salary, and a per diem, but once they got there, the per diem was cut in half and he didn't get but $400 for a $2000 pay. What happened? As in the case with your son, there was no contract. The agreement was "between friends". <Always have a contract!> Make sure it includes the following things:
1: Pay
2: Per Diem
3: Any other travel expenses, like lodging, and very important, airline tickets, bus tickets, etc. - who pays that? IF the band member does not have a passport, who pays for that?
4: Equipment responsibility - what happens if gear is lost or stolen (insurance)
5: Bail out or termination of tour clause. For instance if a band member gets sick on the road and the tour has to be cancelled, or venues cancel dates, etc. Does everyone still get paid in full, or in proportion? This is where the "Act of God" clause goes, too.
6: Who is hiring and all of the contact info
7: Who is paying, and all of their contact info. And the person that is paying or hiring is the one who should sign the contract. If there is no business entity, then the contract must be between that person and the musician.
7: Method of pay and final pay date
8: IMPORTANT: Penalty clause for breach of contract. Something that has phrases in it like" Failure to fulfill contract by x date will make to signer responsible for all costs of recovery, including court fees, attorney fees, travel fees, and an interest rate of 20% per month.
9: And just as important, A copy of the tour book, that includes the dates of the shows, the venues, where the band will stay on that date, how the band will get from one gig to the next and contact info for the venues, the lodging, etc., plus the contact info for the tour manager and all band members, and the band's attorney, if there is one.
10: Who is the person who will get the band out of a jam, if they get into trouble at customs, with the local police, etc.? I would also ask for the number of the US consulate in each country they will be in, too.
Get the contract signed and copies given to all parties at least a month before going out.
11: the band member should have with him at all times, a pre paid cell phone that will work in all countries, to be used in case of emergencies. In addition to his "regular" cellphone, BTW. This is in case his regular phone gets lost, damaged or stolen. It should have the home number on speed dial, and set up so that anyone can pick it up and call home. Should the band member get jailed, sick, injured, abandoned, in trouble with the locals or the local police, he will need a sure way to call home.
My Stepson's headliner got into trouble in the Slavic countries and the idiot headliner barely missed spending weeks in a jail cell. The headliner was on a quest to get laid at every show, except the girl he picked up had a brother who was a local cop, and she said something to the cop brother, and it erupted from there. The local cops were going to throw the whole band into jail and hold them for trial, which would have meant six weeks in a rural former Slovac jail cell where few spoke English. He called us with his international cell and we managed to get the consulate to "Fix" the problem.