I'm going to be getting my bro-in-law a set of new tires for his birthday.
I am planning on getting him a set of Maxxis Advantage tires. (I used them on a trip to NC this summer and I have never been more pleased.)
He is riding a Titus Racer X.
1) Get a matched set 2.1 and 2.1
2) Get him a 2.4 front and 2.1 rear