Waalaikummussalam.
Quran said in (62:9) "O ye who believe! when the call is proclaimed to prayer on Friday (the Day of Assembly), hasten earnestly to the Remembrance of Allah, and leave off business (and traffic): that is best for you if ye but knew!" Yusuf Ali Translation in QuranExplorer
I believe we are not allowed to interrupt khutbah based on several authentic hadiths and the "remembrance of Allah" in previous mentioned verse.
Narrated Abu Huraira "The person who takes a bath then comes to
the Jum`a prayer, then offers the prayer that was destined for
him, and then keeps silent till the Imam finishes the sermon, and
then prays along with him, his sins between that time and the
next Friday would be forgiven, and even of three days more"
(similar hadiths appear in Abu Dawood, Ibn Majah, & Ahmad bin
Hanbal)
It was narrated that Abu’l-Darda’ said: The Prophet (peace and blessings of Allaah be upon him) sat on the minbar and addressed the people, and he recited a verse. Ubayy ibn Ka’b was next to me, so I said to him: “O Ubayy, when was this verse revealed?” But he refused to speak to me, so I asked him again and he refused to speak to me, until the Prophet (peace and blessings of Allaah be upon him) came down (from the minbar). Then Ubayy said to me: “You have gained nothing from your Jumu’ah except idle talk.” When the Messenger of Allaah (peace and blessings of Allaah be upon him) had finished (the prayer), I went to him and told him (what had happened). He said: “Ubayy was right. When you hear your imam speaking, then keep quiet and listen attentively until he has finished.” Narrated by Ahmad, 20780; Ibn Maajah, 1111; classed as saheeh by al-Busayri and al-Albaani in Tamaam al-Mannah, p. 338.
http://www.islam-qa.com/en/ref/45651
There are also discussion about it in the following link.
http://www.crescentyouth.com/board/showthread.php?t=25150
But correcting him in private would be allowed. I am basing this to Al-Asr,103 :1-3
"By (the Token of) time (through the Ages), (1) Verily Man is in loss, (2) Except such as have Faith, and do righteous deeds, and (join together) in the mutual teaching of Truth, and of Patience and Constancy. (3) " Yusuf Ali Translation in QuranExplorer
"in the mutual teaching of Truth" or "recommend one another to the truth" in other translation. You shall not fear if you are in the truth. But it is worthy to remember most of the time the one giving Khutbah are knowledgeable. Respect him and don't belittle him when you correcting him in private. It would be great if you give authentic reasonings and evidence based on Quran and authentic hadith. If he didn't listen to you, it doesn't matter. You have tried.
If there is anything wrong, it was based on my weakness. Allah the All-Knowing knows Best.