Does any one know of a source for the minhag for a chosson to put on his kittel after he is already under the chupah?
Rav Moshe ben Avrohom Mat (1551-1606) in his book "Mattei Moshe" (מטה משה ח"ג בהכנסת כלה אות ב) says, "And we wrap the groom in a white tallis in order to fulfill (Ecclesiastes 9:8) 'Let your clothes always be laundered/white [לבנים]', like the Kolbo wrote in Siman 75. And from this reason the groom is dressed in a kitel at the time of entering the canopy. And it seems to me that a reason for this custom is as is brought in the Midrash, 'We pardon the transgressions of three: king, prince and groom. King as it is written (Samuel 1 13:1), 'Sha'ul was a year old when he became king,' which means like a year old without transgression (Rashi). Prince as it is written (Leviticus 19:32, 33), 'before a hoary head' etc. and next to it is 'And if a stranger sojourn with you' and a stranger who converts is similar to a child who is born. Groom as it is written (Genesis 28:9), 'Then Eisov went to Yishmo'eil, then he took Mocholas.' And was her name Mocholas, surely her name was Bosmas? Rather it teaches that here transgressions was pardoned.' And the Maharam wrote 'and close to this from our tradition is that we find "and shall be clean" is three times in the tradition which represents these three that we pardon their transgressions. Groom as it is written (Numbers 5:31) 'And the man shall be clean from transgression.' Prince as it is written (Samuel 1 26:9), 'That sends his hand against an anointed one of the L-rd and is clean.' A sick person as it is written (Exodus 21:19), 'upon his staff and he shall be clean.'' And since their transgressions are pardonned, therefore he wears a kitel to show the forgiveness by way of (Isaiah 1:18) 'Be your sins like crimson, they shall turn white like snow.'"