There are three reliable ways to stay connected, and each of them serves a different purpose.
Letters and postcards are the most consistent option. A letter costs almost nothing to send, reaches the inmate through regular mail regardless of what is happening with their phone account, and gives them something physical to hold, read, and reread. If you want him to know you are thinking of him and reaching out, a letter is the most direct signal of that. InmateAid's letter service lets you send letters and photos from your phone without using your home address.
Visitation is another channel that does not require a phone account. Being there in person, if you are on the approved visitor list and can make the trip, communicates more than any number of phone calls. Getting on the approved visitor list requires him to submit your name through his case manager, so he would need to initiate that.
On the phone question: the inmate always initiates the call, so he would not receive an inbound call from you. For him to call you, he needs phone access and either a funded account or a facility that allows collect calls. Even setting up that account is something that requires coordination with him first, usually through a letter letting him know to expect it.
The practical starting point if you have not been in contact recently, is a letter. It costs almost nothing and reaches him with no account setup required on either end.