The problem in such cases is a court is usually the only answer. I knew a woman who worked for Gleesons some years ago and she was told to withhold payment to single subcontractors as they would probably go under so Gleesons wouldnt have to pay them. Terrible attitude.
When I used to subcontract from Taylor Woodrow they never paid me or any other contractors one month. They had exceeded their budget for that month so just unilaterally decided against paying anyone. Yes we could have sued but by the time the case hit court we would have been paid as never got anymore work from them again. They had us over a barrel and they knew it.
Is your grandson employed or self employed? I know he has a boss but how is it structured. Either way he cannot just withold money. It sounds like the boss has a cash problem so is not paying everyone what they are owed and using shite excuses to do it.
If he can get the work then definitely go self employed. It's so much more profitable. If he wants to take a day off during the week then so what, provided he is not letting a customer down then it is one of the perks.