Getting paid from agency problem

My son worked for a agency a couple of months ago and has never been paid for the work he had done.

When he started he took all the paperwork they asked him for so he could get paid but when his due payday came nothing was paid.

A few days later son told us that there was no more work for him as where he was doing the work they got rid of the agency for some reason.
So son contacted his boss and was given a excuse of the agency lost his paperwork and bank details so son went and met with his now old boss and gave him paperwork again and that was last Thursday and son was told that the agency will send payment by bacs last week but a week later he still has not beeb paid but was told he would be paid by today without fail.

From day 1 i had a feeling something was not right and with the company where he worked(son was doing some cleaning work) getting rid of the agency made it even more not right.

Also his old boss even said that if he did not get his pay he would pay son himself.
Why oh why would his old boss say that? As i dont know many bosses who would pay wages out of there own money.

The whole thing does not sit right with me.

So what is our next move as son should had been paid back on July 13th.