He may not be feeling specifically for the knots he could just be doing the regular check that all men should do to see if thery can feel any changes. As an aside I had a vasectomy 16 years ago and I cannot feel any knots.
I may have to have a grope of my other half's nether regions later on to see if I can feel any knots.....oh,hold on, he had the other type of vasectomy where they remove a section of the tube and then clip each end,so there won't be any knots,i'm sure he won't mind me checking though :-)