Is it permissible to invoke Allah saying "Ya Hu"
i.e. referring to Allah with He meaning: "He is Allah"?
Single pronouns refer to the speaker, the addressee or the absent. They are not names of Allah in the lingual or the Shar`y meaning because He does not name Himself by it. So, invoking Him with them is nomination, calling and mentioning Him with anything other than His names which is not permissible. It is considered a deviation in His names by calling Him names that He does not call Himself and it is not a permissible supplication. Allah (Glorified be He) forbids that saying:
And (all) the Most Beautiful Names belong to Allâh, so call on Him by them, and leave the company of those who belie or deny (or utter impious speech against) His Names. They will be requited for what they used to do.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.