The weight loss and hair loss could be directly related to poor nutrition as a result of him not eating all of his food. There could be other causes like parasite, exocrine pancreatic insufficiency, etc. But, the first place to start is making certain he is eating all of his food. This is what you can do:
1- Do not leave food out for him at all times.
2- Feed him twice a day and give him 15-20 minutes to eat all of his food, if he does not eat it, remove and offer food at next scheduled feeding.
3- Take him out for walks and stimulate him physically.
4- Make certain no one is giving table food or treats until he begins to eat all of his food.
5- Pick up in your vet's office a food that is called a/d. Add 1/2 a can of a/d to his food; this food taste really good and most dogs love it. Then, gradually wean him off the a/d.
6- Consider getting him dewormed, you can buy Fenbendazole (panacur), give it once a day at label dose for 3 days, repeat after 3 weeks.
7- If after doing the above you do not see improvement have him checked by your family veterinarian.
Hope the above helps, if you have additional questions I will be glad to answer them.