It is always best when you are caught in this type of situation to consult with a criminal attorney in your area as to the "whats" and "hows" that are possible according to your local laws.  What you are concerned about here is the police upgrading the offense in some way and charging him rather than just issuing the citation.  But the citation IS a charge by the police and may be issued in certain circumstances instead of an actual "custodial" arrest, meaning taking him and locking him up.  It seems unreasonable that they could now place him is jail for the same offense, but sometimes the police find other ways to be able to accomplish what they want, like charging him with interfering with an investigation in to where and from whom he got the pot.  Seek legal help.  Good luck.