I used to have one that did that: he was just being a diva. Luckily, he also had a very one-track mind: I learned to ride him forward aggressively and keep his attention on me, me, me....especially when he started to think about spooking.
Be confident and assertive, try to anticipate when he's going to try to stop and don't let him. Keep him moving and give him a few half halts to remind him you are up there. If you have to, do a circle or a figure eight; just something to redirect his attention.
Anyway, that's my two cents. Since your post indicates that this is a recent problem, I assume he's being pigheaded (like mine was), but could be physical as well...some sort of visual impairment for example.