He was right - in Italian Giacomo, the first sylable is stressed.
However, things are really ambiguous. My conclusion in advance:
If Giacomo corresponds to Jacob and Greek was the source, we may assume "Jah-KO-mo". If Giacomo corresponds to Jacob and Latin was the source, we may assume "JAH-ko-mo". "Jah-KO-mo". If Giacomo corresponds to James and Greek was the source, we may assume "JAH-ko-mo". If Giacomo corresponds to James and Latin was the source, we may assume "Jah-KO-mo".
This is a byblical name (Jacob) and there are at least two persons with the name Jacob so that I cannot say of they have really the same name. Here are two text excerpts originally written in Greek.
[Ad Romanos, 9:13] "Sicut scriptum est Iacob dilexi Esau autem odio habui." "As it is written, Jacob have I loved, but Esau have I hated."
This Jacob is mentioned in the Old Testament also. In the Greek text, the stress is on the "o" (it is an omega there). This name is not declined neither in Greek nor in Latin - no special accusative forms are used in this case. This name is not changed in the English translation.
[Mathiew, 10:2] "Duodecim autem apostolorum nomina sunt haec: primus Simon qui dicitur Petrus, et Andreas frater eius, Iacobus Zebedaei, et Johannes frater eius, .." "Now the names of the twelve apostles are these: the first, Simon, who is called Petere, and Andrew his brother, James the son of Zebedee, and John his brother, .."
Here, the name Jacobus is declined in both Greek and Latin. In English, it is translated as James. In Greek, the "o" is omega again (a long vowel) but the stress in Greek is on the "a". In Latin, nevertheless, the stress should be on the "o" if it is long! Thus, I have to assume that Iacobus was pronounced with a short unstressed "o" in Latin so that this name chanhed to Giacomo (the stress on the "a"), Jacques (French) and James (English). Someone may assume the stress on "o" in Latin (as it should be according the rules) and trerefore the stress on "o" in the Italian Giacomo.
It would be interesting to me if these names (Jacob and James, Latin Iacob et Iacobus) are really the same in Hebrew.