First things first...
you need a grooming table. You can get one for less than $70. If you're cringing at the expense, consider all the time and trouble you saved by not brushing your dog for the first year of his life.
There is no brush, comb, electric rake, or gadget that is going to help until you are able to
keep the dog in one spot. With a table, you don't need to hold him down. He can't just walk away. He's on a table; yes, he might try to jump, but you can hold him with one hand and brush with the other. Better yet, have a friend hold his head while you work.
Once you have him on the grooming table, go slowly. Again, think of all the time you saved by never brushing him - you have plenty of time in the bank. Start by just running a pin-brush over him and pulling out as much dead coat as you can. As you've observed, there is so much there that it will be easier going for a while. Brush the surface and pull that dead stuff out. Cookies and praise.
Next, get yourself a copy of this book:
The Illustrated Guide to Sheltie Grooming. I know, I know -
but Tofu Pup, I'm already out the cash for the table! I hear you. But it's a great book, and it walks you through the necessary tasks of correct coat/skin care. You can even learn how to trim your Sheltie like a pro, once you're ready for that.
Eventually, you will get to where you can line-brush him with water. It is imperative that Shelties be brushed/combed
to the skin on a regular basis. That means that if you part the hair, you can see the skin - and if you can't see it yet, you brush and comb and pick until all of the dead undercoat is out. The skin needs to breathe; it needs fresh air and stimulation to stay healthy.
It is a shame that your dog did not receive an early education about grooming. It may be that, as a result, he never really takes to it. But with patience and kindness (and by limiting his options with a table), I think you can get him to where he will accept it. But you need to get serious about caring for his coat.