My dog is balding on his neck. Is it from his collar or something else?

Poor Sharky! The area of his hair loss is certainly consistent with irritation from his collar. From the pictures, it looks as though Sharky may also be wearing a flea collar, and these products can actually cause a lot of skin irritation in more sensitive dogs. Other causes of the hair loss could be related to hypothyroidism, mange, hormonal imbalances, among other issues. At this point, consider taking all collars (flea or normal collars) off of Sharky and use a harness to walk him until the hair loss resolves. You can also try bathing Sharky in a gentle oatmeal based pet shampoo since this can be soothing on the skin. Make sure that Sharky is regularly receiving a high quality flea and tick preventative, such as Nexgard, Bravecto, or K9 Advantix II in case a flea allergy could be contributing to this issue. If he starts to itch and scratch, if the hair loss worsens or does not begin to improve in the next few weeks, Sharky will need to go to his vet for further evaluation. I hope this helps and that he's feeling better soon!