This is the best suggestion so far , I think. I have one and it is small and slim , very light but will cut what you need to cut. One the few times that I am forced to wear slacks or suit pants ( which I detest with a burning passion.. ) I choose this knife , I cannot stand having lumps in my pockets when I have to wear those terrible , terrible excuse for pants.
TBH, anything without plain plastic/metal/G10 handles would look fine with a suit and tie, like micarta, semi-precious material inlay, polished/jigged bone, stag, etc., or anything with a Damascus steel blade, because Damascus does look quite class.
As 6thtexas stated above, the A.G. Russell Titanium lockback (or is it lockblade?) knives are great for in-pocket carry - small, well built, very sharp, and they come with a nice leather sheath that helps protect the pocket from the knife and the knife from stuff that my be in your pocket. I like them a lot, carry one often and give them as gifts occasionally. If, when you say formal, you want a fancy knife for a fancy suit that could also be a conversation piece in a fancy suit crowd, and you have the discretionary funds, then William Henry has a lot to offer. And I mean no disrespect when I say "fancy". The William Henry knives could be described that way but I see them more as incredibly artistic, imaginative, precision built, very useable knives - including Deferg10's in the above post. Many also come with a leather carry sheath. Though I've not had to wear a suit for a long time, I have treated myself (my wife often suggests I buy my own birthday present) to a couple over the years. You might also like their E6 model. Depending on the direction you want to go, there are a number of good custom makers on Blade Forums also. Mike