I adopted a 10 month old Chiweenie, and he is all skin and bones, his old owners were feeding him cheap food.

Is he still young enough that he needs puppy food, or is there a specific brand I should buy? Also what habits can I get into that can help him gain weight?

There are several puppy foods that have no wheat and start with chicken is the primary ingredient, I'd go with one of those. Have you wormed him? I'd have the vet check him and bring uup the topic of worming, never know what else the previous owners skipped. Some swear by this http://bluebuffalo.com/compare-pet-food/?gclid=COSL267O_q4CFcsAQAodXRfR3A

He can go on adult food now but I would keep him on puppy food until his condition improves.
Puppy foods generally contain a lot more calories and nutrients than adult foods and they are for (obviously) growing puppies rather than adults maintaining weight.

I won't get into brands as there are so many different opinions about them, but when I am selecting a food I look at the ingredients on the back... If meat is the first ingredient listed you know it's the highest %. If things like what, corn meal or any grains are listed first then they are foods to avoid, you don't want to highest percentage ingredient to be a filler with little to no benefits.

Good food doesn't have to cost the earth either, I pay $11 for a 6.5lb bag of food and it's considered in the higher range of the average priced foods... Mine do excellent on it and it lasts them a whole week (I own 3 toy dogs).

Ballenger makes a good point about worming, an infestation of worms can cetainly cause weight issues as well.