My son was charged Jan 10 with possession of marijuana with intent to sell. The police also took his cell phone. Are they allowed to make calls on his cell phone and send text messages pretending they are him or is this against the law

A cell phone belonging to an alleged drug dealer is evidence. They can do pretty much anything they feel will provided evidence including lying.

Yes, this is done all the time,

Yes, this is legal and is done every day by the police to catch more drug dealers. Great invention, the cell phone and texting.