While he's happy to follow me and is calm, he will stop and "look" for several seconds at "something". I do one of two things, based on my previous experience. Wait for him to walk forward, which he will, or keep the lead snugged up and release when I ask him to come forward, which he will, but sometimes not right away. My concern is which is the best way to handle this before it gets worse or when we are under saddle on the trails and the same thing occurs.
He is known to back up but he hasn't come across anything yet that derails him when in hand. My trainer gave me some tips on derailing his backing up under saddle (yield hind quarters, make it a sideways movement).