Shot a deer like this a couple of years ago. Cut it open and it was some kind of infection. Couldnt find any type of thorn in there but that is what we suspected. Broke off inside and became infected. As for the meat, it was fine.
Looks like a good deer to harvest if you are managing your place.
I'm in total agreement with you Soap, I will bet he is 6+.
I am no expert, but sometimes on older animals, as certain organs start to fail, they lose serum and it gathers at the lowest point on the body.
I really think that animal needs to be taken.