This is going to make the chest of the sweater a little smaller, probably about by about an inch across the front and another inch across the back. If that's ok and the sweater seems like a good fit for your husband, just continue with the pattern up to the shoulder shaping. When you get there, cast off 16sts at the beginning of the next 6 rows and leave the center 50sts on a holder or spare needle (16x6)+50=146.
You'll need to work the same pattern for the back. You can tell how many sts you cast on by counting the sts in the k2p2 rib at the beginning of the front. Also, look at the way the cable panels are set up and count the k3p2 ribs on the sides to make sure you're doing the back the same way as the front. It's always a good idea to go through the pattern and circle the all directions for the size you're making.
As long as the smaller size fits, you should be ok. Keep good notes from now on for all the steps you're doing, like the shoulder shaping.