My 2 year old dachshund has a gastro intestinal infection and won't eat. What I can offer him that won't make matters worse?

Going without eating a few days won't hurt. See the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171

You could try the chicken and rice bland diet in the sticky.