Aah! I can vouch for weather changes and spooking. My normally mild mannered sweetheart turns into a psycho pinball machine every March for about 3 weeks. He's so unpredictable, I figure if I can stay on him those 3 weeks, I'm good for the year.
The solution definitely is to get his attention on you. When my horse is being a nut, I accomplish that by riding every single stride. Every 6 strides or so, change what you're doing. Flexion, shallow serpentines, leg yeild, shoulder-fore, haunches in, ground poles, whatever you need to keep his attention.
Now to the good part, the cantering. Just today I tried a new game with my horse. He has a nice canter, but I've been having problems getting him to extend his stride. The game we played was canter-trot-canter. The transitions alone will get your horse into a lighter frame, but you can really find gold if you refine it a bit.
Pick your spots and cue him at very specific times. Setup for the transition as if you were riding a dressage test. A few strides out, think about what you need to do to properly cue him, give him the necessary half halt to let him know a change is coming, and then transition. As soon as you feeling him start to fall back on his forehand, bring him back to trot. Again, don't allow him to fall into it. Transition down with balancing half halt into a nice energetic trot. If he tries to fake you out, remember he had all that energy to spook. It's about time he uses that same energy to give you a nice trot. As soon as the trot is established, start all over again. Find an exact spot...etc. AFter a few transitions, you'll feel him stepping under himself more and he'll start to round up into a nice frame. If you're reins get more floppy with each transition, you're on the right path. (sidebar - notice how little the reins factor into balancing a horse once you've got him collected)
I did that exercise just today on both leads with my 6 year old. I was dumbfounded to find that his canter feels completely different when he is truly balanced. He felt like he was floating. His stride was suddenly adjustable. I was able to get 4 strides where earlier in the ride I was getting 6, but without a change in speed. Instead of having to worry about this shoulder, that leg, where his head was, it was one continuous working piece. That's the only way I can think to describe it. It was amazing.
Everyone's got great suggestions on this site, so try all the ones you like and see what works for you. And don't worry, the spooking should stop once it warms up a bit.