If I were to trust your guess from the actual context, I think I know exactly what the phrase would mean. The verb I am thinking of is more often written 「キメる」 rather than 「決める」, but that is not a rule, so here I go.
「キメる」 has a slang meaning of "to take drugs". Thus, the sentence would mean:
"Isn't he on some kind of drug?"
「決める」 has a few important meanings that have nothing to do with "deciding". Read definition #11 from コトバンク which says:
"To drink and/or eat. To dine. Or to consume illegal drugs."