Even after you put money on the phone, there are several reasons calls might not be coming through.
The most common issue is that your number may not be on his approved call list. Most facilities require inmates to submit a list of approved numbers before any calls can be made. If your number was never added, or was added recently and has not been processed yet, he physically cannot call you regardless of how much money is on the account.
The facility itself may also be in lockdown. Phones get shut off during counts, incidents, disciplinary periods, or administrative lockdowns. If the unit is restricted, no one is making calls until it lifts.
It is also worth checking which account the money is on. There are two separate systems at most facilities: the inmate's phone account and their commissary or trust account. Money on one does not transfer to the other. If the funds went to the wrong account, the phone still shows zero balance.
On his end, access to the phone depends on his schedule, his housing situation, and whether there is competition for limited phone time in the unit. Not every inmate has easy or consistent access even when everything else is in order.
The most reliable way to get a message to him right now is through a letter or postcard. InmateAid can get something to him within about a week without your personal address appearing on the envelope. Let him know the money is there and ask him to confirm your number is on his approved list.