What should I feed a stray dog? Currently he is fed milk twice a day.
Is this enough for him or does he need any other nutrients?
Most adult animals are lactose-intolerant; milk is not a good idea.
Dog food would be the obvious recommendation. Is there a reason you can't do that?
Absolutely not.
As Keshlam noted, lactose intolerance is a factor for canines, but it's worth understanding that dogs are omnivorous and require a number of nutrient sources for proper health and survival. These include:
Water. While there is some in milk, the ability to make use of it depends and it's unlikely enough.
Proteins. Milk actually has pretty good protein levels, but the amino acid profile may not be sufficiently providing all essential amino acids.
Fats. Depends on the milk, but it's also not the best kind of fats anyways and the volumes are very low.
Carbohydrates. Very low in milk, not a significant source of carbohydrates.
Vitamins and minerals. Milk has some, but not all, so supplements would be critical.
The ASPCA has good information on canine dietary needs. The short story on that, though, is that dogs need a much more varied diet than milk. Puppies, for a frame of reference, usually start weaning after 4 weeks. There's a point where the dietary needs of the puppy cannot be met from the milk provided by their own mother. Based on that, you can extrapolate that milk from another species is even less likely to be enough.