If your mailing came through InmateAid, the return address on everything he received is InmateAid's office in South Florida, not your address in Australia. He can write a letter back to that address, and InmateAid will receive it, scan it, and post it to your account so you can read it. The cost to retrieve a return letter is minimal.
The process is the same whether you are in the next town or on the other side of the world. He writes back to InmateAid, you log in and read it. No international postage, no customs delays, no privacy concerns about your address circulating in a county jail in Pennsylvania.
If he wants to call you, international rates from Allegheny County Jail would be very high using the standard jail phone system. That is worth knowing before setting up any phone account for an overseas number.
For now, a return letter through InmateAid is the simplest and most cost-effective path to making the connection.