I am interested in adopting a dog but it says he is being treated for heartworm?

I saw a dog that my family is interested in adopting online. The ad said he is up to date on everything, yet they said he is completing heartworm treatment that will be ending this month. could he have more potential problems down the road?

Talk to their vet. Depending on the severity of the heartworm (and I assume he was positive and this was not just a preventative treatment), there can be limitations. The following quote came from the link below.
“Surviving dogs may need lifetime treatment for organ damage. The sooner the disease is diagnosed and treated, the better chance for a good outcome.”

Pedigree Dogs ad shot 1000 FPS using the Phantom camera