Grandson has a "wart" inside is lip...what causes this.

Unlikely the issue: The more likely finding in that location is a mucocoel. Mucous producing glands within the inner lining of the mouth/lips can have their pores broken by the trauma of eating.This traps mucous in cystic collections that may have a wart like appearance. Usually , I have an ENT surgeon remove these.

Few things: A soft tissue lesions can be caused by a few different sources. It may be viral or benign growth, for example. See a dentist for an exam. It may be nothing significant, or may require a simple biopsy. Either way, they will help identify the etiology and possible treatment options. Best of luck!

Papilloma\fibroma?: I've never seen a "wart" intraorally. What I do see often is benign growths such as fibromas and papillomas (benign growths), mucoceles (liquid filled vesicles from blocked salivary ducts), etc. Some patients also mistakenly confuse normal anatomic structures with abnormalities. Have a dentist look at what you are describing for an accurate diagnosis and appropriate treatment only if needed.