The 55105 mag fits the bottom metal that has the mag release lever on the bottom metal itself.
By the OPs picture I don’t see a mag release lever. I am pretty sure he needs the newer Axis style mag that has the lever molded into the mag itself.I believe it is a 55232.

https://www.ebay.com/p/26031092688

Longdude, does your son know if this is what the mag looked like? And answer for sure, does the rifle have the mag release lever on the mag itself or on the bottom metal?