Okay so you shrunk his favourite sweater now what? Unshrink it! Fill your sink full of warm water and a cap of baby shampoo.Let it soak a while and work it through your sweater.
Then after a little while remove the sweater from the sink and gently squeeze it out.Do not rinse.Take a large towel and lay the sweater flat on it.Roll the towel up with the sweater inside.It will absorb extra moisture,so your clothing is damp,but not soaked.
Then get another dry towel,and put your sweater on it.Gently stretch it out as it lays to dry.Continue to do this until it is back to it’s original size.Allow it to air dry on the towel.Then voila! Problem fixed,you won’t even have to tell him about it.