Unfortunately, he has no legal recourse for the paychecks.
The thing is that you got a court order to get back his stuff from his father house. That's really all you need to win small claims. No judge will believe that the exhusband has nothing of the son.
But if you want to try another way, this might work.
i don't know the full story of what happened. I can only guess that this is what happened.
Your son was kicked out by his father and he came to live with you.
When he went to get his stuff back, his father refused to let him back in. If so, this is an illegal eviction and you probably did not know this. Your son can call the police and tell them that he has been illegaly evicted as a minor and they will escort him back inside the house to get his stuff back.
If your exhusband says that's he moved out months ago, tell the police that you didn't know he was illegaly evicted until recently, which most minors dont know.
For the police, it's one thing if you tried to get your stuff back and them claiming they don't have any and another if someone has been illegaly evicted and there is no way to legally evict him since he is a minor.
They will most likely escort him back inside the house. Even by the slim chance that they don't, file an illegal eviction police report, then sue him in civil court and use the illegal eviction as evidence. As a result, the burden of proof falls on the exhusband due to the illegal eviction. So you won't need receipts or anything but they will be helpful. And you also have the court order to get back his stuff as additional proof.
Small claims court maximum is 10,000.
If you are claiming more than than you will need a lawyer.
Also even if exhusband lets you in, don't forget to file the illegal eviction police report and if you want sue him in small claims court for anything he destroyed, got rid of, ect. Once again, the burden of proof falls on him due to the illegal eviction
The worst case scenario is that he doesn't get back any of his stuff.