I am not able to view your tracking video. You might try Susan's suggestion of using two six foot leashes hooked to any of the links on each side of the fur saver. The leashes go under the front legs and you handle a leash in each hand like a plow mule. This technique will give you better control and help to pull his head down. You might also try doing a large scent square first. Stomp in an area about 4'x4' square and put food throughout the scent square. Somebody suggested a pea sized bait. I use slices of hotdog about the size of a nickel. What are you using for bait? You don't want it to be too visible or to smell too strongly. If your dog's food drive is really strong, try letting him track for single pieces of kibble. Think of the scent square as a track that goes nowhere. If he focuses on the square well and eats the food, move on to the track and do a track about 30-40 feet long. If he keeps going out of the square and is not scenting the trampled grass and eating the food, take him away and put him up and try it later, witholding food until the next tracking session, ideally, later that day. Another way is to lay three short tracks parallel to each other. Make each track just a little bit longer than the prior track. The idea is that he get to practice a short track, gets rewarded and praised, repeats with the second track, etc. This gives the dog three chances to practice instead of one, so the practice effect should come into play.
You might not want to wait so long after you have him sit before starting the track. It could be that you are capping his drive, which builds his drive, and then he is so drivey, he isn't focusing. Play around with that. Maybe just let him start to tracking without sitting him first.
Since your dog has good food drive, you might want to put a toy at the end of the track. Or, don't give him the food jackpot at the end. Instead, just leave a single piece, praise and release, and have some food set aside off of the track.
As for videoing, I'd put the camera on a tripod at a 90 degree angle to the track so you can get a side view. This way you can easily see if the dog is keeping his head/nose to the track. Plus, you can see how you are handling the line better. It won't show you as much if you are using two six foot leashes, but when you start using a longer, single leash, you'll be better able to see the lines and if the tension is constant or going slack at times.