"Call him tomorrow."
Translation:Ruft ihn morgen an.
'Ruft' would be plural imperative, right? 'Ruft (ihr) ihn morgen an.' So the sentence tells multiple people to call one person tomorrow - that seemed like a strange idea at first, but I can see it now: you might be telling the parents of a sick child to call the doctor in the morning.
No. "Morgens" means "in the morning". And the time reference has to go to the end of the sentence: "Ruft ihn morgens an = Call him in the morning". Also correct would be "Ruf ihn morgen an", as it is not clear from the english sentence whether this is addressed to one or several people.
Morgen ("tomorrow" in this case) is not a noun but an adverb and therefore it doesn't have to be capitalized and the preposition (am) is redundant. If you want to say "Call him in the morning" I think that would be Ruft/Ruf ihn am Morgen (an) -or- Ruft/Ruf ihn morgens (an). The an is part of the separable verb Anrufen which means "to call (someone) over the phone", without the an it's Rufen, which is just "to call (someone)".
Are you asking why "an" is after morgen? That's because anrufen is a separable verb. Separable verbs are verbs with prefixes that get detached when conjugated (except in cases like when it is used in the infinitive or past participle). The prefix tends to go to the end. For example, "Ich rufe ihn sofort an." (I am calling him immediately). "Rufen" by itself means "to call". "Anrufen" specifically means to call someone over the phone.