Your previous 56cm frame was also a Cannondale? Did it fit? If it did then I can't imagine a different size frame can also fit.
Of course if your 56 meant you were using a 14 stem with spacers under it and a seatpin up to its max (or beyond) then you may have scope to go up a size.
Like I said, a 1cm shorter stem is all he needs to get exactly the same reach. In fact 1cm is so little he may not even notice it.
99% of bike fitting is getting the seat in the right place relative to the bottom bracket and putting the bars at the right place relative to the seat. I'm 6ft tall and I can easily do this with pretty much any 52-60cm frame, using normal components. The reach on bike frames doesn't really vary that much. The top tube lengths change, but smaller frames also have steeper seat angles which erases some of the difference.
On a Six13 the 52cm size has a 74deg SA, an 12.1cm HT, and a 53.5cm TT. The 60 has a 72.5deg SA, a 19.2cm HT, and a 59cm TT. The difference in seat angle is eq to ~1.5cm in TT length, so I still have 4cm real difference in reach.
On a 52 I'd need a pretty long stem (~130mm) tilted up (18+6deg) with maybe 20mm in spacers, and a seatpost with some setback. On the 60 I'd want a shorter horizontal stem (~90mm), no spacers, and the seat a little farther (1.5cm) forward on the rails. Note that I'm getting the seat and bars in *exactly* the same position, so the fit will be identical. Beyond that it is mostly esthetics... and yes I prefer a 55-56cm frame for that reason.