Fox, 30, had been without a club since leaving Norwich City in June and had been training with the U's.
He spent a season at Colchester between 2009 and 2010, making 21 appearances, before joining the Canaries, where he featured 72 times.
Hewitt, 20, has played 16 games for Town since signing from Macclesfield.
He moved to Portman Road two years ago and had a spell on loan at Gillingham last season, playing 20 times.
"David adds experience to a young squad," Tony Humes, boss of the League One side, told the club website.
"He fits our profile of how we want to play the game and will be a good addition to the squad for the coming months."
Colchester United have signed midfielder David Fox on a three-month deal and right-back Elliott Hewitt on a month's loan from Ipswich Town.