Can my son be charge if they found some kind of drug bag from the truck he borrowed?

Yes, the question is if they can be succeed in the prosecution against your son. He needs an attorney to defends himself.

Yes, the police can charge your son with drug possession if they found drugs or drug paraphernalia in a vehicle he was driving.

They can and often will. Anyone can be charged. It is one thing to be accused, it is another matter entirely to have your guilt proven beyond a reasonable doubt. They are probably charging him under the theory of "constructive possession" meaning that they knew the drugs were there and had reasonable access to them. The fact that the truck wasn't his would be good circumstantial evidence of his innocence. There may also be grounds to challenge the search and seizure of the drugs. Only an experienced criminal defense attorney that regularly handles drug crimes could fully answer that question and what your options are and likely outcomes after thoroughly evaluating the evidence and police reports. Consult with an attorney as soon as possible and do not talk to the police in the meantime.

He can be charged, but the case against him is not as strong as many cases they bring. Legally, possession must be "knowing" possession to be a crime. However, a jury may infer knowing possession from the fact of possession. You need to find experienced representation in your area because this could go very well, or very badly.

Absolutely, especially if he forgets he has the right to remain silent and cops himself out. Remember, no matter what he says it will be twisted around and used against him later.