明日、彼と会うことになっているので、何か連絡があったら伝えるよ。

、、。

I'm supposed to meet him tomorrow, so if he gets in touch, I'll pass the message on.

Literal

Tomorrow, him [with-と] meet [nominalizer-こと] [dative-に] become-is [because-ので], something contact [subject-が] exist [conditional-たら] convey [assertive-よ].

A natural, conversationally packed sentence. ~ことになっている for the arranged meeting, ので for the causal 'so,' たら for the conditional 'if.' 伝える ('to convey/pass on') is the standard verb for relaying a message. 何か連絡 = 'some sort of contact,' using 何か as an indefinite modifier — 'any contact at all.' The final よ adds reassurance.