Bless you for giving Thaddeus a forever home and welcome to the Daily Kitten! You will find lots of very experienced people here who love cats and will be very generous with advice.
I think this is very typical behavior for a young kitten. If you can imagine what you would feel like being taken from your mom and siblings at an early age to go to a noisy shelter, then moved again to a home, you can understand why Thaddeus might be upset. He misses his mom and siblings. He believes you are the only one who can meet his needs for food and comforting which is why he keeps crying for you and trying to get to you. It's actually a very good survival skill in the wild. :)
As far as I know, having diarrhea is not an indication of overfeeding. He may have an upset tummy or a bug of some sort. Have you had him checked out by a vet? It is very important that you take him in to be tested for Feline Leukemia and Feline HIV which are communicable to other cats (not humans). A vet will test him for worms and treat him. Do you know what vet care, if any, he received at the shelter?
The danger with diarrhea is dehydration. Kittens can go down hill very quickly if they get dehydrated. Make sure he is getting enough fluid. You say you have been feeding him formula. Is it KMR (Kitten Milk Replacement)? Cow's milk can give kittens diarrhea because they don't digest it well.
You might want to consider letting Thaddeus sleep with you. It would be a great comfort to him and you can get some sleep too. (Our two cats sleep with us.) It's actually quite nice to have a little foot warmer on top of the covers. Some folks' cats sleep on their pillow or cuddled into their necks. If that's not your cup of tea, then make him a soft nest with towels or a little blanket. You might want to get him a small stuffed animal to cuddle up against.
Other more experienced folks will post here so please keep checking back!
I look forward to hearing more about Thaddeus. When you have time, you can upload a photo of him and use it as your avatar here.
Posted 3 years ago by Kitten Whisperer #