My ex boyfriend 4 years ago was diagnosed with brief psychotic disorder.two months ago we broke up and since then i've noticed a change in his behaviour, not being himself. can this be a relapse; is a relapse possible in brief psychotic disorder?

A "brief psychotic disorder" could be a reaction to drugs or a toxin, and even a viral event. You may be seeing a repeat of the original event secondary to introduction of prior items, or he may possess a chronic psychiatric disorder which is now exacerbating. Either way, get medical attention, confirm a diagnosis, and get treatment as appropriate.