How much bigger you have to go? If it's like 4" just add it at the bottom. Cut the front and back pattern about 1/2 way up in the middle, and open it 1" on each piece. You shouldn't have any probs doing that. for more than 4" i cant help, as more seams would have to be changed .Measure the bottom of a shirt that fits him and go from that.

I have to add 6"........the waist on the pattern only goes to 60". thanks.

If the pattern fits in the chest area and only needs to be larger in the waist area, I would simply add to the side seams.
Add a piece of tissue to the side of the pattern piece (plastic grocery bag works well to) gradually enlarge from under the arm to the hem line. Two inches on each piece will give you a total of six inches. Does the six inches include some ease? If it doesn't make sure you add around 3 inches for ease, otherwise he will not have any room for movement.

Hope this helps.

will i have to increase the sleeve as well to fit the new inches....yes the 6 inches includes the ease. thanks.