I would make a smaller size and lengthen it. All patterns have a line where you are instructed to "lengthen or shorten here".

You could get smaller size that would fit him in widith and alter the length as described above. That would be the easiest.
The other way would be to do a similiar adjustment to the legs of the pants. lets say you want to take them in 2 inches. On the pattern piece on each leg make a 1/2" fold the length of the legs-(pulling it in a total of 1") secure the pattern piece fold with a pin. You would also have to adjust any separate waistband pattern piece, if any, to correspond.
It really be easier to do the first suggestion of getting a smaller size and lengthening, but if you have the pattern and can't return it I think the second way would work.

Sally

Great! Thank you both, not sure why I didn't think of that! :)