Hello Kthree, welcome to Psych Central.

Seroquel is one of the newer antipsychotics, link here Psych Central Reviews - Seroquel (quetiapine fumarate)

As far as I am aware yes antipsychotics are usually stepped down gradually but it sounds like with the urgency of the matter that he has to come off it right away. I would have thought that another antipsychotic would be prescribed instead of taking it away completely though.

An antipsychotic medication would not cause psychosis. Psychosis happens when a person becomes seriously distressed and an antipsychotic is used to keep it at bay.

I wasn't aware that methadone/seroquel interact. I would urge you to contact the clinic yourself for more information and check whether your son needs another antipsychotic in the meantime. Also contact his psychiatrist and ask these questions, they are pretty important! Good luck.