When Walter goes home for lunch with Jem and Scout, Atticus treats him like a man. They talk about "grown up" farming stuff. Atticus began the discussion because he knows it's polite to talk to/with people about what they are interested in. Walter will never do well in school, so academics would not be a good topic. Atticus stuck to what Walter knew well. That shows he has respect for little Walter. This shows that there is no "class system" in the Finch household. The link proves this.
Also, when Scout made fun of his eating his lunch with syrup all over it, Atticus was not the one to jump down her throat. Calpunia stepped in. Atticus remained in the background and then carried on their conversation as Scout was taken off by Cal.
Atticus's treatment of Walter is a great deviation from Scout's perception that "He's not company....He's just a Cunningham." Atticus treats Walter as a guest in the house, and converses with him about topics such as agriculture, which he knows that Walter is familiar with.
Despite the fact that the Cunninghams are one of the poorest families in town, he is nonetheless treated as if he is no different than any other middle-to-upper-class guest. This gesture shows Atticus's refinement and gentlemanly qualities. A less cultured person would not have been so inviting.
Atticus treats Walter as a guest and while Walter is a Cunningham, just like how Atticus does not discriminate for race, Atticus does not discriminate on the basis of caste. This shows Atticus' kind heart and compassion he has for others as well as his strong morals he upholds.