What can I do to get 83 year old to eat? He drinks 2 Ensures each day and nothing else. Has no appetite. 5ft10in and weighs about 100 lbs.

You don't say WHY your 83 year old is refusing to eat. I would suggest a medical evaluation to explore this. Many dementias progress to a complete loss of appetite. And you would be amazed, absolutely amazed at how little the body can live on (yes, with weight loss, but it just goes on).
If you have already discussed this with the elder's doctor, and have followed any ideas he or she may have, you basically are looking at an elder who has chosen not to eat. Try to see that there is a bit of exercise, even if up in a chair. Being this emaciated and without good nutrition can lead to bedsores which can be a death sentence for the elderly. It may be time to discuss palliative or Hospice care.
I wish I had more answers for you than "see the doctor" because I sound like the "Take two aspirins and call me in the morning joke" often enough here. So many things just don't have a "fix it" in the case of advanced age, but at least making the doctor the first stop will insure there isn't something that's being missed.

You may need to have him evaluated by his Dr for swallowing issues, as that could prevent him from wanting to eat. Have you tried giving him pureed foods? It also could be that his teeth are bothering him, or a sore in his mouth, making chewing difficult. Or it just could be that his body is shutting down,(preparing for death) and doesn't need food any more. Probably need to get him checked out by his Dr. so you know what you're dealing with.

Might be time to also get hospice involved, if he's 5'10", and only 100 lbs.