There is a machlokes rishonim as to what the status of birchas erusin is. From the Rambam it is clear that it is a birchas hamitzvos. The Rambam (Hilchos Ishus 3:23) rules:כל המקדש את האשה, בין ע"י עצמו בין ע"י שליח, צריך לברך קודם הקידושין הוא או שלוחו, ואח"כ מקדש, כדרך שמברכין קודם כל המצוות – “Anyone who is mekadesh a woman, whether he does it himself, or he does it via an agent, a berachah must be recited before the act of kiddushin, either he himself should recite it or his agent, and then the kiddushin should be performed, just like what we do by all mitzvos.”
The Ra’avad argues on the Rambam and says: We first perform the act kiddushin and then recite the berachah, as it requires the consent of another party, and we are worried that maybe the woman will refuse, and it will turn the berachah into a berachah levatolah. However, the Ra’avad just argues on when to recite the berachah, however, he agrees that it is a birchas hamitzvos.
However, from the Rosh in Kesubos (1:12) and Tosfos HaRosh (7a) it is clear that the birchas erusin is a birchas hashvach. The Rosh writes:וברכה זו נתקנה לתת שבח להקב"ה אשר קדשנו במצוותיו והבדילנו מן העמים וצונו לקדש אשה המותרת לנו ולא אחת מן העריות – “This berachah was enacted, to give praise to Hakadosh Boruch Hu for the fact that he commanded us and separated us from the nations of the world, and commanded us to be mekadesh a woman that is permissible to us and not one of the aroyas.” See the Ritva (7a) who also learns it is a birchas hashvach, although he learns like the Ra’avad that the berachah should be recited after fulfilling the mitzvah.
Based on the above, according to the Rambam and Ra’avad that the birchas erusin is a birchas hamitzvos then the chosson should hold the ring, and according to the Rosh and Ritva who learn that it is a birchas hashvach then he doesn’t need to.
