In-flight he will get a pass at the snack basket (maybe two passes depending on the crew). The basket usually has fruit, Biscoff, pretzels, Kind bars, some sweet things (brownie bites or caramel), and some savory (chips, popcorn, puffed rice).
How old is your son? Some clubs (those with self-service bars) require 21+ to enter. I think all the clubs in ATL have a full service bar so it may not be an issue.
Generally, lounges have some buffet food with hot and cold items. A couple of soups and some sort of sweet thing. They sometimes have novelty items like hot dogs or local baked goods. The F lounge is the best in ATL if he has time to go all the way to F (presume he will be arriving in T, A, or B).
There isn't a SkyClub in IAH but he would have access to the KLM lounge. Delta flights are in terminal A whereas the KLM lounge is in terminal D. There is an air side train he can use but it is a good 15 minutes from the Delta gates so may not be worth the hassle. That club is pretty small but has some OK food and drink options.