I actually understand this, kinda like I have one dog that would rather lay on the floor than in the bed..lol. Seems odd to us but they like it. If he really likes the garage try to set up an area just for him. Lots of water, a fan on the area where he will lay down and bc of his age a nice soft blanket to lay on to help with any pressure sores he might get from the hard pavement. Make sure you check on him frequently and let him out to potty and try to still get him to play...moving will help those joints and bones too. If you notice him panting excessively you might want to bring him inside and allow him to cool off. Good Luck.
if he likes the garage, I see no reason not to let him be in the garage. Some dogs find houses too "busy". I have a dog (a newfoundland) who simply WON'T come into the house. She likes her place in the backyard with her kiddie pool, her own deck, and her big shady trees.
the garage is most likely concrete floor? they lay on the cold floor and it feels better than carpet that traps their body heat.
we have partial hardwood and partial carpet and all the pets are on the hardwood!