As-Salam aleikum wa rahmatullahi wa baraketuh,
correct, Isaiah 9:5-7 refers to Prophet Mohammed (pbuh), since I’am engaged in this issue, I can confirm it to you with 100 % certainty ! There are many evidences also which shows very clearly that the child described in Isaiah 9 is actually the Servant of God mentioned later in Isaiah 42 and which is none other than Prophet Mohammed (pbuh) ! It is sad that some Muslim scholars and apologists takes the Jewish side and interprets Isaiah 9 as a reference to Hezekiah, not becasue this is a resonable explication, but it is a good argument to disprove that is a prophecy about Jesus (pbuh).
Take care, and salam
Ahmed (Poland, Warsaw)