@karenamara, according to this article about fluphenazine decanoate (Modecate), dizziness and drowsiness or fatigue can be side effects of the medication.
https://www.medbroadcast.com/drug/getdrug/modecate-concentrate
When experiencing different and concerning side effects, your partner should really contact his doctor as soon as possible to report the side effects and let them know the change in where the medication was administered.
Karen, is he also noticing the side effects? Would he be willing to call his doctor?