Oh yeah, as shockneck said, no way will any 5.3 media boot on your system.
When you boot AIX it loads a mini install kernel and then tries to configure devices.
No way will the 5.3 even load the config / install kernel on a 7248.
5.3 requires at least a chrp system with firmware late anough to support 5.3 and the 7248 is not a chrp system.
Get it booting a 5.1 diag CD and then go find a copy of AIX 5.1 install media to install AIX - if it will even support 5.1!
I cannot find any docs for the 7248 any more so 4.3.3 could be your limit.