We have a 3 mo (about to 4 mo) old Cairn Terrier puppy who had a great appetite at first, but has stopped eating. He will have a few bites if we stand right beside him or put a little bit on the mat beside his bowl. We take the bowl up after 30 minutes and he doesn't seem to care - he just wants to play (which is very fun)!

We are giving him 1 cup in 1/3 servings of Purina Puppy Chow everyday (this what the breeder used as well). He is very active and playful, but I am worried he is not eating enough. He was the runt of the litter and is already mostly house trained.

Any suggestions?

Sounds to me like a classic case of an over fed dog. What does the vet say? In the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171
There is a link to evaluating your dog. Unless he is excessively thin, just cut back what you are feeding. Talk to the vet next visit about it.

Stick to the Purina 1. The less switching around of chows you do the better. The ingredients may vary, but the chows are all formulated to have the same nutrients. The large assistance dog school I just raised a puppy for has all their highly valuable dogs eating it, or at least the adult version.

Thank you, LabMan. I could definitely see him being a "spoiled" little pup. :) I will consult with the vet to see how much we should lower the feeding by...

Thank you for your help!