If th PRL were wrong, he couldn't use the phone. It determines which tower he can connect to, and if he can't connect to a tower th whole phone stops working.
Downloading an MMS uses the APN settings. which are probably wrong. Get the correct ones from your carrier (each carrier uses different settings - and maybe more than one set) and make sure yours match. Or bring the phone into the carrier and ask them to set the APN data correctly.