Altering men's slacks is one of the easiest alterations to do. Many stores only carry even sizes in slacks. If you are an odd size, buy one size up and take in the waistband 1". Taking these to a tailor can be $15-30. If you know how to use a sewing machine, you can follow these easy instructions and do it yourself. All you need is: sewing machine, iron and ironing board, thread to match, the slacks, a sew kit and ruler.
To see if your slacks are made for easy alteration, turn slacks inside-out. Look at the back of the waistband. It should look like the slacks above. The back seam is sewn in one continuous seam through the waistband. The waistband is not sewn down. Notice the shadows under the waistband. These are made for easy alteration. You can take these in or let out. See how wide the seam allowance is? If your hubby has gained weight, there is plenty of room to let out. I will be showing you how to take them in. As you scan the photos below, don't be afraid of waistbands that you may have never sewn before. The alteration is very easy.
Don't let this inner construction scare you. Bring both seam allowances together. Open up top corners. These flaps may or may not be tacked down. If they are sewn down, find stitching and unsew with your seam ripper. Since the seam was pressed open, now press flat with seam allowances together.
With slacks on a flat surface or ironing board, position so that you can visibly see the back seam. The pants will be on your left and the seam allowance on your right. Take your transparent ruler and align the existing seam under the 4/8" line on ruler. Draw a line along the ruler edge using the white dressmarking pencil from your sewing kit through the entire waistband. (To take in slacks 1" you need to take in seam 4/8".)
Once you have marked the waistband, now align ruler so that it extends from the white line drawn on the waistband to the existing stitching just as the crotch begins to turn. Draw along edge of ruler with white pencil.
Prepare seam to be sewn by pinning layers together. It is especially important to pin through the bottom edge of the waistband. Feel with your fingers to align both layers and pin through seam as shown.
Before you sew, change presser foot to be an open-toed foot. You want to be able to see the white line you are sewing on. Thread machine in a color to match the slacks. Backtack at the beginning of the seam, at the fold of the waistband, and at the bottom of the waistband (where pinned). Continue sewing down the white line. Eventually you will be sewing over the existing stitching. Continue sewing through the crotch and backtack again.
Now you must take out the original seam using the seam ripper. First, carefully take out a few stitches where the white line meets the original seam (not shown). Next, remove stitches at the top of the waistband.
If you are lucky you may find a running stitch that undoes all at once. It will stop running where you removed the few stitches at the bottom of the seam. If stitch does not undo easily, carefully continue using seam ripper to take out seam.