Hello, Linda -
Without knowing at which prison your son is incarcerated, so that I or someone else volunteering to answer questions on Fixya.com can study the kinds of paid services available to inmates and what companies provide them, it is a challenge to answer you.
My understanding of tablets available to prison inmates is that they have limited functions and limited electronic access, unlike the kind of tablets you or I may use outside a correctional facility. There are companies which provide all sorts of goods and services to inmates for a price (a high price, in my opinion.)
For example, JPay is one such business. This is a link to the JPay web page which has information on the kinds of tablets available to inmates who are incarcerated in prisons with JPay services and a list of what can be accessed on the tablets:
Even if JPay is not providing goods and services to the prison where your son in an inmate, the tablet options through the company doing business with the prison will probably be similar.
Note: The funds available for your son's tablet access may have already been used up, which he may mean by "restricted." The electronic services for inmates in one of the prison systems in the US seemed very expensive to me when I investigated them for a family member last month.
For an explanation specific to the tablet your son has access to, you will need to contact the company providing the table and access services. If you can speak on the phone with someone and ask your questions, I believe you will gain a better understanding.