If he's doing it exclusively on lead, then it sounds like he's being protective of you. Once, someone mentioned as a "btw" that they were also pregnant (which explained it!), so I'll ask you if anything has changed (not necessarily related to your fecundity!). They are very sensitive to you and your needs, and if something has changed, expect your V to recognize it and react to it. If not, then you need to reassert your authority here, he somehow believes you are in need of protection...by keeping a tight lead, telling him "NO!" when he exhibits this, stand still with him until you reassert your position in the pack, and reinforce him settling down with a "Good calm, good boy!", and then proceed.
In spite of the so called "Fear period", which I never encountered btw...you'd need to still address this now so it doesn't flourish into a full blown case of fear or aggression growling.