Thank you for taking-in this little Squirrel and putting him under your loving care! One of the conditions that comes to my feeble mind with what you are describing, McCarthy; is relatively low dose poisoning with Bromethalin. It is relatively unlikely in a park setting but still possible, I suppose. There is no antidote for Bromethalin but low dose poisoning may result in some degree (or possibly nearly full) of recovery. Higher dose Bromethalin poisoning usually results in hyperactivity, seizures and quick death so IF this is from Bromethalin, it was not a high dose poisoning!
It may be as you included in in your list of possibilities that this Squirrel is elderly and your willingness to make his remaining days safe and comfortable are very kind and admirable and all that is necessary or possible.
To cover another possibility that you also mentioned; I would also like to suggest that you immediately begin treating him for MBD using Henry's protocol (
https://www.henryspets.com/what-is-m...-bone-disease/ ). It is a simple protocol to follow and is quite effective and if it's not MBD, the treatment will do no harm while if it is MBD, it could very well save his life and return him to normal functioning!
It is a commonly stated that wild Squirrels do not get MBD and that this is a condition found only in captive Squirrels but I have always doubted the logic of this. My feeling on this is that just as you mentioned in your thread, wilds can easily have some of the same issues as captives that could predispose them to MBD such as inadequate Vitamin D or inability to obtain a protective diet that would ordinarily contain adequate Calcium. Also, for Squirrels in a park setting where they may be fed by humans with nuts, treats, and human food; they may very well be getting excessive phosphorus and even if they had what would ordinarily be an adequate and protective calcium intake; the Calcium

hosporus ratio may be so low that MBD will develop anyway!
Regards and thanks for your devotion, care, and love for these Little Squirrels. Again, welcome back to TSB!
SamtheSquirrel