We do not reccomend stopping formula, it is always best to let them wean themselves regardless of how long that takes. The first solid food that should be given is a good quality rodent block. Many of us feed HHB's, this block has been made especially for squirrels and will provide all the nutrients your pet squirrel needs to stay healthy and avoid MBD. Formula and rodent block are the best assurance against a captive squirrel getting MBD. Block is given when eyes open to get them used to it. They will sniff it, bite it, and sometimes pee on it, eventully they will eat it. You want them to be eating it and eating it well before any other solid foods are offered.
Behaviour can be pretty exclusive to the squirrel. Males may have a tendency to show more aggression when their testosterone kicks in, however when females come into season it's no walk in the park either. It is natural for a squirrel to "wild up" as they get older, after all they are wild animals and that instinct is pretty hard wired in them at birth. Unless there is a physical reason that would prevent them from being released and surviving in the wild, they should be raised to be released back into the wild where they were meant to be. They do not make good "pets" and should not be considered as one.
Just from what I can see of the thumb nail pictures my best guess on age would be around 6-7 weeks of age.
Questions: Was eyes closed or open when you got him?
What is the formula (type) you are feeding?
I'm including below the forum for the healthy diet
*chart and also the site for HHB's. ....And
to TSB!
* (Chart can be enlarged by clicking on it)
http://thesquirrelboard.com/forums/showthread.php?44440-
Healthy-Diet-for-Pet-Squirrels http://www.henryspets.com/