You certainly have a gorgeous little boy there Jess, Izzy has done you proud!! Like Diane, we dont usually put a mare with a new foal back with the 'herd' of ladies. We either graze in hand for a few days or turn them out alone during the day in a small very grassy paddock. They will then join mares who also have foals, we dont put them with other mares. Or if the mare is the first to foal, she will occupy the 'mare and foal' field alone until she is then joined by others once they too foal.
Also if it is still cold winter weather where you are, I would be wanting him to be stalled at nights until the warmth of Springtime - if you put Izzy back with her other friends, will you be able to catch her to bring her back in each evening (given her present attitude!)? Or perhaps she will come happily for her food?