Awww sweetie! Yes it is most likely the lithium, he has probably got toxicity. That is when the levels are too high in his system. It can do weird and wonderful things.

I'm glad he has gone to the ER, they will do a Lithium level blood test to see what level he is at, it takes a while to get back btw, so don't worry when it's taking so long. Our son had a reaction to it too, and his results were bizarre, when they took his blood in hosp it was actually below the therapudic level but when he had had it done two days before it was way above. So the pdoc explained that he must have been going in and out of toxicity throughout the day etc.

Hang tough, I know that's easy for me to say but hopefully some answers come out of this. If he does have to go to the other hosp, hopefully they find something he is better on. If it ends up not being the meds they may add one for the paranoia and delusions.

Hugs to you! I know how hard this is!
Sally, Mom to Jacob 9 (Bipolar, ASD, Expressive/Receptive Language disorder, Anxiety, Tourettes Ben 10 and Max 3