Find an obedience class (one that uses positive training) so you can learn how to train your dog and have someone who can give you advice on handling any problems that may crop up.
Meanwhile, does your dog like food? Toys? Use whatever HE considers a good reward (my one dog is a chowhound who will do anything for food, the other will work for his favorite toy). To teach sit, hold the treat/toy over his head, above his nose and move it back - 9 times out of 10, the dog will sit. When he does it every time you move the treat/toy back, add the word 'sit'.
You can also use a treat or toy to have him lay down. Hold the treat in front of his nose and lure him into a down. Once his elbows hit the floor, "GOOD boy!" and treat. As he gets the idea, add the word 'down'.
You can use the Gentle Leader to walk, but you should also work on teaching him to walk nicely on leash, either in 'heel' position or, at least, not pulling. For me, treats worked. I would give Michaela lots of praise and a treat every time she walked next to me in heel position (on my left side, her ears parallel to my leg). As she got the idea of heeling, I gradually faded the treats to one or two, but kept up the enthusiastic praise (because she likes being told what a good girl she is.
Good luck with your boy! He's a handsome fellow!