Aw, what a precious baby.
I want very much to lie to you because your grief is quite clear. But I think you really want to know, and I also know that once you raise one of these little gems, you will want to do it again, and I don't want to see anyone go through the hurt twice.
Squirrels have very exacting nutritional needs. They require quite a bit of calcium in their diet - their skeletons need it to provide for the flexibility and gymnastics that are part of their life.
There are a few things that will block calcium uptake, and one of the big ones is phosphorus. Now phosphorus is necessary in the diet, but what you are looking for is a balance of about 2 parts of calcium for every one part of phosphorus. This all means that no only do you have to make sure they get their calcium, you also have to make sure they don't get foods that are super high in phosphorus, or it will cancel out the calcium. And guess what - of course the highest phosphorus levels are found in the things they love the most.
Corn, sunflower seeds and cashews are all EXTREMELY high in phosphorus. Corn has one part calcium and
45 parts phosphorus. Sunflower seeds have one part calcium to 9 parts phosphorus. Cashews are 1 to 12. Pumpkin seeds are 1 to a whopping 27. Seeds, grains, and nuts usually have the highest levels of phosphorus.
We solve this diet dilemma by feeding either a specially made squirrel food that one of our members manufactures, or by feeding "rat block" which is an extruded diet made for rats, which are fairly close to squirrels in their dietary needs. These foods have the proper balance, and are combined with healthy veggies and occasional fruits to make up the total diet. Nuts are fed in very small quantity as treats only. Even among nuts there is a huge disparity - almonds are not all that bad, being 1 to 2 calcium to phosphorus. Hazelnuts are about the same. But pine nuts - they are 1 to 36. So even with treats we pick and chose.
This info is not that well known. They make commercial squirrel foods from corn and sunflower seeds! Most vets don't know, no pet store employees know - you need to really mine for this info if you are not lucky enough to have found a board like this early on.
In the absence of enough calcium, or the presence of too much phosphorus, the squirrel will begin to pull calcium from it's own bones to satisfy the need. It causes the bones to become very brittle. Once that has happened, you will finally begin to see symptoms - trouble walking sometimes, and very often, seizures. Because they hide their sickness so carefully, most people do not see problems until the situation is very advanced. His age is just when it hits, too - the baby formula has plenty of calcium but once they wean from it, the danger time begins.
BTW, kitten formula is not a good milk sub for them - and I am sure you were told to use it - VETS will tell you this for God's sake. It has too much protein and not enough fat. If this ever happens to you again, first, run to a health food store and get fresh goats milk and some vanilla yogurt, for a good temporary formula can be made from those items. Then come here, and we will hook you up with a place to get a formula specifically designed for baby squirrels, and we will help you every step of the way so you never have to go through this again.
I am so sorry about Buddy. My Mister P is literally the most important being in my life, and even thinking about losing him makes it hard for me to breathe, so I can imagine how you feel. I am so, so, sorry.