Hm, my guess is the game is resetting the monster's stats everytime you enter a new map or put him in your pack and release him - because it spawns a new creature each time.

Instead of ShadowKeeper, try Near Infinity, and locate and change the familiar game files. It will be in the CRE directory, and will be FAMXXX.CRE, where XXX would be CAT, PSD, IMP, RAB, etc. There's a "25" version, and that's for BG2:TOB familiars.

Hmm, still not working. :/ The CRE files are in the override folder within the GOG Baldurs Gate folder right? I tried Near Infinity and its saying the FAM files are updated to my liking, but in game, they aren't updated for some weird reason.