Most the post from new owners when a beardie is not eating well after a few weeks they find out it has coccidia or worms. Coccidia is often found in bearded dragons in minimal amounts and pose no problem, but stress such as shipping, new enviroments and other changes such as separation from siblings and others can cause a overload of them to develop.
They are usually fairly inexpensive and can answer many questions that we can only second guess on a computer. But it is advised by most experience owners to make that your first priority after buying one, especially if it does not eat well.... our babies eat about 50-70 appropriate size crickets daily, plus about 1/4 cup salads and veggies and they are not overweight, but nice size in lenght.
Also make sure the temps are correct and he is heating up properly to digest 95-105 in basking area (we have some that like it warmer, but no less and around 80F in the cooler area