if he signed for his stuff and then just could not hall it but did not have someone at the store mark how many he took. He might have a problem in small claims court. If the store has a receipt that he bought 36 pieces of plywood and he does not have anything showing he did not take that many I cannot see a judge awarding it to him.
We didn't get to that part yet, but proper prior preparation for Small Claims Court would be to take pictures of his truck, & document its carrying capacity. Then he weighs one piece of the plywood in question and demonstrates, through calculation, to the judge that it is just plain not possible for his truck to carry 36 pieces of the plywood, or that the load would not be safe to carry.
This plus his testamony is probably enough to establish that he did not take all of the wood at one time.
Then the problem of proving that he took a second load of wood falls back on Home Depot, and from what he says so far, they have no paperwork suggesting anything other than the departure of one load of plywood.
The other thing which can work in his favor is the discovery and subpoena process the court has. He needs to write down the name of everyone he has talked to at Home Depot (both locally and at the main office) and, if he fills out the correct paperwork, the court will issue subpoenas for all of them to appear for the trial.
In many instances this is sufficient to get the defendant to settle a case. He is on his own time, but Home Depot is going to have to pay all of these witnesses to appear. The total cost of this is probably higher than the cost of the load of plywood.
If a witness does not appear for the trial in resonse to a subpoena, he can get an arrest warrant issued. Now no one is going to go out and chase these people down if they don't show up, particularly if they are out of state, but if they ever get a traffic ticket in the state, they will spend the night in jail. This is serious food for thought.
When dealing with large companies, it is frequently advantageous to point out to the local manager that the cost to him of a trial will greatly exceed that of a fair settlement.
Any kind of signs on his truck anywhere near Home Depot will be frowned upon by the court. Try the legal remedy first, then the signs.