My dog is a rottweiler of 4 yrs old. He was poisoned and at the vet's for almost 3 weeks. He weighed 180 pounds and is now home and down to 95 lbs. Can you suggest anything to help him gain his weight back?

He was poisoned? The hair on the back of my neck is standing up... and I can't imagine what 3 weeks at the Vet costs!

Didn't the Vet recommend specific food, a specific diet?

I'd be afraid of anything too high fat at this point.

You really should talk to your vet about what kind of food would be best for your dog, he is a large/giant breed dog and there would be a very real concern for his bones, you will also need to take into consideration the type of poison, was there any internal damage? How will it affect his nutrition absorption.
When my German Shepard was recovering from whip worm and diet of rice and de-boned chicken or beef hamburger was recommended.

180? Are you sure about the weight, or did you mean to type 108?
That would be a weight loss of 85 pounds in 3 weeks or roughly 28 pounds a week.
I've never seen weight loss that extreme.
The question is probably best answered by your vet, depending on the effects of the poison we don't know what lingering organ problems may still exist.
If his liver, kidney, digestive tract, etc were damaged and still healing then your best option is to ask your vet what diet is best for him at this point.

First I would normally agree that chicken is great for dogs. However, we don't know what was done to your guy and if there is any lingering damage, likely is a long road to recovery. I am a bit surprised the vet did not give you a list of does and don't. Different poisons damage different organs.Betting LadySam made a good call.