My Bubba had a beautiful coat, then he got matted in the undercoat -I foolishly cut it out & it has never grown back - it's been almost 2 yrs

He eats high quality food with missing link added - but maybe I'm not using enough ML - he gets 1/4 tsp a day - he weighs about 4 1/2 lbs.
I've attached a few pix of him. The coat thru the body is thin, with only one layer - the undercoat is about an inch long at most. I believe he has a silky coat.
I have spent hrs reading, I think every thread on YT about oils, long coats, etc. - but I'm still confused and have some questions.
1) If the coat isn't growing could it be because it's dry??
2) How do I know if Bubba's coat is dry - does it look dry?? it doesn't feel that way to me, but I'm not sure what it would feel like
3) How do I find the right product? -- I want to promote growth & healthy hair - should I go with a cream or an oil to achieve this result??
4) Should I add an additional supplement to the Missing Link?
5) Should I consider changing shampoos also?? Should I go to something with oatmeal?? I use a shampoo with these ingredients: Coconut shampoo base, aloe vera gel, essence of lavender, essence of mint, chamomile extract, sage extract, nettles extract, rosemary extract, wheat protein extract (natural deodorizing agent), tea tree oil, vitamin E, vitamin C.
From reading other threads, I have narrowed products to treat his coat down to these choices:
Plush Puppy Seabreeze oil
Emu or Almond Oil
Vellus Satin Cream
Pure Paws cream
Right now I'm leaning towards the Vellus because I've heard hair will grow with this. Have you used this?? Does oil help promote hair growth? Or should I use the Vellus & an oil??
Sorry for all the questions, just want to get this corrected in my sweet boy