Like a man, the way to a dog's heart is through his stomach! Most dogs are foodies, and you can use this to your advantage.
When I start to leash-train a dog - and I admit I haven't worked with a 120-pound one - I put lots of good-smelling treats in both pockets. It's amazing how much more a dog likes me that way. Then I start attaching a twelve-foot clothesline rope securely to his collar or harness, and take him to an open area (such as an empty parking lot). I let Rover roam around to the end of his lead and get used to being restricted from going further. When he comes back in my direction - whether I call to him or not - I give him a bit of a treat. (All he needs is a taste - not a full snack.) Then I start walking the other way. Rover roams around, gets to the end of the lead, works his way back to me, etc. I do that for a little while, just to get him used to having a lead and paying attention to my presence.
The next day I'll cut a foot off the lead, and another foot the next day, until it's down to the regular six-foot length. Every time the dog comes to me, I reinforce the behavior by giving him a taste of the treat.
When I put on a regular leash, I'll let him practice with it a couple of days, calling his name frequently and rewarding him when he pays attention to me. Pretty soon Rover is thinking, "I'm getting this woman trained; she gives me a treat whenever I look her way."
If Rover doesn't know how to sit, I teach him to sit on leash by holding that bit of treat near his nose and moving it back so that he has to sit in order to get it. That needs practice every day. (Just a couple of minutes three times a day might work.)
When I finally start walking him on leash seriously, I have the smelly treats in the pocket on the side I want Rover to be on. I hold one in my hand, and encourage him to walk with his nose close to the treat. I reward him if he does this for just a bit. I have to catch him doing what I want; when he's in the right place, he gets praised and treated. Eventually, I can teach him to stop when I do and to sit - right where the treat is. After he gets used to it, the rewards can be fewer and farther between, and praise will do the trick.
Seven isn't too old to teach a dog, as long as he is physically and mentally healthy. Hold the leash so that it isn't wrapped around your wrist or anything, because, at his size, if MegaRover takes off running for some reason, you could be hurt if you can't let the leash go.
If you can take Rover to a few classes, he will watch other dogs walking on leash, and that may help him, too. Dogs are pack animals - they learn from one another.
Hope this helps a bit.
(I just read the suggestion about the Gentle Leader. I use Gentle Leaders all the time! Dogs don't like having things on their noses, so you have to do the same kind of reward training to get them used to it. You also need to learn to adjust it correctly so that it allows Rover to breathe, pant, drink water, etc. - not too tight, not too loose. The advantage of a Gentle Leader or Halti is that you're guiding Rover by his head, not pulling on him by his neck. Where the head goes, the rest of the dog will follow.)